Compiled by the Social Cohesion and Identity Research Programme of the Human Sciences Research Council in association with the Africa Genome Education Institute Published by HSRC Press Private Bag X9182, Cape Town, 8000, South Africa www.hsrcpress.ac.za © 2006 Human Sciences Research Council First published 2006 All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers. ISBN 0-7969-2119-9 Copy editing by Karen Morrison Typeset by New Leaf Design Illustrations by R Nanni and Robert Hichens Cover design by Richard Mason Print management by comPress Distributed in Africa by Blue Weaver PO Box 30370, Tokai, Cape Town, 7966, South Africa Tel: +27 (0) 21 701 4477 Fax: +27 (0) 21 701 7302 email:
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The Crossing Over Pilot Teacher Trainer Research Programme (a project of the Human Sciences Research Council) was a week-long course held in Cape Town and attended by 33 teachers with representivity from all nine provinces and from rural, urban, private and state schools. We chose the name Crossing Over as it suggests not only the transmission of knowledge from one to another, but the shift from one state of knowledge to another too. It is also the act that chromosomes perform in the event of meiosis. Crossing over is fundamental to change. Crossing Over was designed to cover the basic content necessary for teaching the key concepts of comparative functioning, relationships and the development of change, otherwise known as evolution, in molecular biology. There was a special emphasis on lesson planning skills and a series of exciting visits to appropriate environments. The teachers had access to the best that is available in the country both in terms of facilitators and sites (a glance at the Acknowledgements reveals this). This book is a compilation of the material that was developed for the Crossing Over Pilot Course for the GET (General Education and Training) curriculum and the FET (Further Education and Training) curriculum. A further book, Reading Scientific Images: The Iconography of Evolution, has also been produced by the project for educators interested in the cusp between art and science and more specifically in reading scientific images. We hope that through this publication, Crossing Over is able to add value and bring satisfaction to educators beyond the team of the course facilitators, the group of participants and their learners. We thank the Royal Netherlands Embassy and the Human Sciences Research Council for the financial support and the research resources that they respectively contributed to the project. Sandra Prosalendis Project Coordinator: School Curriculum and Scientific Literacy Project Social Cohesion and Identity Research Programme Human Sciences Research Council
HSRC Workbook
Preface
Acknowledgements Putting together the Crossing Over initiative was a collaborative effort and required the expertise and the imagination of the team named here. Thank you to Mr Utando Baduza (HSRC), Ms Pam Barron (HSRC), Ms Colleen Dawson (Consultant HSRC), Dr Edith Dempster (UKZN), Dr Helen De Pinho (UCT), Mr Luvuyo Dondolo (HSRC), Mr Adrian Hadland (HSRC), Ms Cathy Hastie (SABC), Dr Wim Hoppers (RNE), Professor Wilmot James (HSRC), Mr Richard Mason (UCT), Professor Tony Morphet (UCT), Mr Kanthan Naidoo (Gauteng Education Department), Professor John Parkington (UCT), Ms Sandra Prosalendis (HSRC), Dr Jaishree Raman (MRC), Mr Trevor Samson, Ms Gillian Warren Brown, Ms Lynne Wilson (HSRC) and Ms Jean Witten (HSRC). The contributions of the UCT MicroBiology Department, Kirstenbosch Gardens, Central Methodist Mission in Cape Town, the MTN Science Centre, the South African Museum, the Fossil Park, Sivuyile Tourism and Information Centre and Sivuyile Teacher Training College in Gugulethu, and Women Unite are also acknowledged. We thank the teachers listed below for their enthusiastic and considered participation: Mr Thabo Msutu (Mzontsudu S.S. School, King William’s Town), Ms Kuzeka Gecelo (Lingelethu J.S. School, Cala), Mr Anthony Pandaram (Westville Boys High, Wandsbeck), Mr Barry Booysen (Ebenhaeser School, Wepener), Mr Ezekiel Moyaha (Tidimane Middle School, Mogwase), Ms Nandipha Mapukata (Blorhweni J.S.S, Ntabankulu), Mr Setshwaro Mokgethi (SaHeso Intermediate School, Roodepoort Farm), Mr Philemond Nkuna (Noto High, NW Province), Mr John Visagie (Intermediate School Keimoes, Northern Cape), Mr Dumisani Dlodlo (KwaDomba High, Nongoma), Mrs R. Ramgoolam (Greytown Sec. School, Greytown), Mr Stephan le Roux (Stanford Lake College, Haenertsburg), Mr NP Sebone (Marumofase H. School, Indemark), Mr Thomas Jafta (Newslands East Sec. School, Marblerary), Mrs G.N Links (St. Boniface H. School, Kimberley), Mr Johnny Witbooi (Bridgton Sec. School, Oudtshoorn), Mr Nicholas Smith (St Boniface High, Kimberley), Ms Chairmaine Stalmeester (Bridgton H. School, Oudtshoorn), Mrs Nozuko Phakela (Fezeka Sec. School, Cape Town), Mr Thomas Mathew (Somavugha High, Mahwelereng), Mr Ashley Engelbrecht (Simunye High, Cape Town), Mr NE Nyawose (Durban Natural Science Museum, Durban), Mr Retsisang Moreku (Mhwayi Primary, Kabokweni), Mr Siyanda Mcwango (Gordon Memorial High, Dundee), Mr Benjamin Chipulu (Janjo High, Gopano), Mr Lesetja Seopa (Mapule Sec. School, Bakone), Mr Frans Bodigelo (Mmadikete Intermediate, Brits), Mr Amos Rangata (Emadwaleni High, Soweto), Mr KW Kgopane (Sango Combined School, Laersdrif) Mr N Kamteni (Sophumelele Sec. School, Cape Town) Mr Thabo Tsunyana (Sinako Sec. School, Cape Town) Ms Alticia Klaasen (Villiersdorp Sec. School, Villiersdorp). The author and publishers would like to thank the following people and organisations for photographs and micrographs used in the publication: The University of KwaZulu-Natal Centre for Electron Microscopy; Mike van der Wolk; Harcourt Education.
Introduction
vi
The Natural Sciences curriculum
viii
Using the workbook
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Part 1: Understanding Genes & Inheritance Contents Overview 1. The cell 2. Measuring very small structures 3. The cell cycle 4. The chromosomes and cell development 5. Mitosis 6. Inheritance 7. Selection 8. General principles of reproduction 9. Sexual reproduction Solutions to activities
1 3 4 9 14 16 19 23 32 36 38 44
51 52 55 57 60 62 70 75 94 95
Part 2: Introducing evolution Contents The Life Sciences curriculum Overview 1. Charles Darwin and the voyage of The Beagle 2. Darwin’s theory of evolution 3. What evidence supports Darwin’s theory? 4. Present day evidence of evolution 5. Genetics and evolution Resources Solutions to activities
HSRC Workbook
Contents
Introduction Did you know? The Human Genome Project (1990 – 2003) was an international effort which aimed to identify the 20 000 – 25 000 genes in human DNA, find out more about the chemical composition of DNA, store the information in databases, and investigate the ethical and legal issues related to their findings. Although the project is finished, the information that scientists have found will be analysed for many years to come. You can find out more about this project online at: www.ornl.gov/sci/techresources/ HumanGenome/home.shtml
The Human Genome Project is one of the greatest scientific efforts of the current age. As a result of the HSRC’s interest in this project, we ran a teacher-training project to help teachers become aware of the importance of the Human Genome Project and to increase their understanding of genetics and the ways in which characteristics are transferred or inherited from one generation to the next. This short course was developed to provide teachers with the knowledge and understanding required to do so. Part 1 of this workbook is designed for Senior Phase teachers. It forms the basis for the more advanced concepts needed in the FET phase which are covered in Part 2: Introducing Evolution. An understanding of evolution is very important for teachers, particularly as this is a new topic in the school curriculum. We believe that understanding the theory and examining some of the supporting evidence, will lead you to agree with the famous evolutionary biologist, Theodosius Dobzhansky, who said ‘Nothing in Biology makes sense except in the light of evolution.’ When we first trialled this material, we ran a week-long course for 35 teachers from all over South Africa. We were very lucky to run the course in Cape Town, which is within a famous centre of evolution, the Cape Floral Kingdom. The itinerary below shows you how the course itself helped teachers to raise questions and develop their own understanding of the evolutionary process. Day 1 Kirstenbosch
experience the incredible variety of plants in the Cape Floral
Botanical Garden
Kingdom
University of
extract DNA from onions
Cape Town
use models to discover how hereditary information is stored in
– MicroBiology
DNA, and then translated into protein molecules
Laboratory
investigate simple hereditary characteristics such as blood groups, widow’s peak, attached ear lobes, and hair on the middle finger joints.
Some of the teachers were amazed to find out that each of us inherits an equal number of genes from our mother and our father, and that our children inherit equal numbers of genes from their mother and father. They asked questions like this: ‘I have 7 brothers and sisters, and all of them inherited equal numbers of genes from our mother and father. Why do we look different?’ It was quite a challenge to answer all the questions! Day 2 West Coast
view 5-million year old fossils being excavated.
Fossil Park
The fossils were of large, sturdy giraffe-like animals all lying where they had been buried five million years earlier. For most of the teachers, it was the first time they had seen fossils, and it made them aware of changes in the Earth’s surface, and changes in the life forms that have existed on the Earth. It also made them aware of the long history of life on Earth, because five million years ago is just the other day in geological time.
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HSRC Workbook
Day 3 South
activities on the process of natural selection
African
trying to find patterns of similarity and difference in flowers
Museum
visit the museum to look at similarities in the bone structure of vertebrate forelimbs.
By then, some of the teachers had realised that evolution was not something mysterious and dangerous, but a very good way of answering the ‘why’ questions in Biology. Some of these questions are:
Why is there so much duplication in organisms that occur in different parts of the world? For example, each continent in the southern hemisphere has a different species of large flightless bird – the ostrich in Africa, the emu in Australia, the moa in New Zealand (now extinct), the rhea in South America, and the cassowary in New Guinea.
Why do we find fossils of organisms that no longer live on Earth? How do characteristics pass from parents to their children?
Why do children resemble their parents?
It has been estimated that there are about 20 million species living on Earth at present.Why do we find such enormous diversity of life on Earth today?
Why do we find patterns of similarity and difference within the diversity of life?
What has happened to those organisms?
Why are there so many different species filling the same niche on different land masses?
A few teachers made all the links, and realised that evolution is about variation in the genes, and that through the process of natural selection, certain individuals have a better success rate in breeding and therefore passing on their genes to the next generation. It all seemed simple and so logical because the experiences were built up in this way and teachers could see that evolution progresses through natural selection in each successive generation.
Crossing Over: The Basics of Evolution
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The Natural Sciences curriculum The Revised National Curriculum Statement (RNCS) is the current policy document on education. The RNCS places outcomes at the centre of learning and teaching, but it also specifies content topics that must be covered, and those must constitute 70% of the teaching time in each grade. The remaining 30% of the time is available to extend the core knowledge, or to introduce content from local contexts. The core content in Natural Sciences draws knowledge from four main areas: l Life and Living l Energy and Change l Planet Earth and Beyond l Matter and Materials. The Natural Sciences Learning Area is constructed around three learning outcomes: Learning Outcome 1: Scientific Investigation The learner will be able to act confidently on curiosity about natural phenomena, and to investigate relationships and solve problems in scientific, technological and environmental contexts. Learning Outcome 2: Constructing Science Knowledge The learner will know and be able to interpret and apply scientific, technological and environmental knowledge. Learning Outcome 3: Science, Society and the Environment The learner will be able to demonstrate an understanding of the interrelationships between science and technology, society and the environment. Natural Sciences educators are required to construct learning programmes which will help learners to make progress in all three learning outcomes throughout the GET band. The learning outcomes are further described by a set of Assessment Standards that specify the levels of achievement within each learning outcome in each Grade. The Assessment Standards for Grades 7 – 9 are shown in Table 1.
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Table 1: Learning Outcomes and Assessment Standards for Grades 7 – 9
Learning Outcome
Assessment Standard
Grade 7
Grade 8
Grade 9
LO1: Scientific Investigation
1. Planning investigations
Learner plans simple tests and comparisons, and considers how to make them fair.
Learner identifies factors to be considered in investigations and plans ways to collect data on them, across a range of values.
Learner plans a procedure to test predictions or hypotheses, with control of an interfering variable.
2. Conducting investigations and collecting data
Learner organises and uses equipment or sources to gather and record information.
Learner collects and records information as accurately as equipment permits and investigation purposes require.
Learner contributes to systematic data collection, with regard to accuracy, reliability and the need to control a variable.
3. Evaluating data and communicating findings
Learner generalises in terms of a relevant aspect and describes how the data supports the generalisation.
Learner considers the extent to which the conclusions reached are reasonable answers to the focus question of the investigation.
Learner seeks patterns and trends in the data collected and generalises in terms of simple principles.
1. Recalling meaningful information when needed
Learner, at the minimum, recalls definitions and complex facts.
Learner, at the minimum, recalls procedures, processes and complex facts.
Learner, at the minimum, recalls principles, processes and models.
2. Categorising information to reduce complexity and look for patterns
Learner compares features of different categories of objects, organisms and events.
Learner applies classification systems to familiar and unfamiliar objects, events, organisms and materials.
Learner applies multiple classifications to familiar and unfamiliar objects, events, organisms and materials.
3. Interpreting information
Learner interprets information by identifying key ideas in text, finding patterns in recorded data, and making inferences from information in various forms such as pictures, diagrams and text.
Learner interprets information by translating tabulated data into graphs, by reading data off graphs, and by making predictions from patterns.
Learner interprets information by translating line graphs into text descriptions and vice versa, by extrapolating from patterns in tables and graphs to predict how one variable will change, and by identifying relationships between variables from tables and graphs of data, and by hypothesising possible relationships between variables.
4. Applying knowledge to problems that are not taught explicitly
Learner applies conceptual knowledge by linking a taught concept to a variation of a familiar situation.
Learner applies conceptual knowledge to somewhat unfamiliar situations by referring to appropriate concepts and processes.
Learner applies principles and links relevant concepts to generate solutions to somewhat unfamiliar problems.
1. Understanding science as a human endeavour in cultural contexts
Learner compares differing interpretations of events.
Learner identifies ways in which people build confidence in their knowledge systems.
Learner recognises differences in explanations offered by the natural sciences and other systems of explanation.
2. Understanding sustainable use of the Earth’s resources
Learner analyses information about sustainable and unsustainable use of resources.
Learner identifies information required to make a judgement about resource use.
Learner responds appropriately to knowledge about the use of resources and environmental impacts.
LO2: Constructing Science Knowledge
LO3: Science, Society and the Environment
Crossing Over: The Basics of Evolution
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In an outcomes-based framework, the content is the vehicle that educators use to facilitate learners’ achievement in each assessment standard. The Natural Sciences curriculum statement provides a list of core content topics that must be covered, but it provides very little detail about the depth and breadth of each topic. This means that educators must have good subject-matter knowledge in all four content areas of science, as well as access to a wide range of resources so that they are able to construct learning experiences that are interesting and valid in Natural Sciences. Each content area is further divided into two or three sub-strands, giving a total of ten sub-strands in the learning area. Teachers are required to draw on appropriate content from all of the sub-strands to build the assessment standards and ultimately the three learning outcomes for the Natural Sciences learning area. This workbook aims to help you build your own understanding of topics that fall within the content area Life and Living. Although the Natural Sciences learning area statement does not specifically mention the term evolution, many of the content topics relate directly to evolution and the processes involved therein. Table 2 shows the content areas that relate directly to evolution, together with an explanation of how they relate to evolution.
The National Sciences curriculum
Content area and sub-strand
Knowledge statement
Link with evolution
Life and Living: Biodiversity, change and continuity
Unifying statement: The huge diversity of forms of life can be understood in terms of a history of change in environments and in characteristics of plants and animals throughout the world over millions of years.
Change in environments is the engine that drives change in life forms, or evolution. The history of life on Earth is the evolutionary history of life.
South Africa has a rich fossil record of animals and plants which lived millions of years ago. Many of those animals and plants were different from the ones we see nowadays. Some plants and animals nowadays have strong similarities to fossils of ancient plants and animals. We infer from the fossil record and other geological observations that the diversity of living things, natural environments and climates were different in those long-ago times.
This content statement introduces learners to the idea that life has a very long history (millions of years) and that fossils tell a story of changing life on Earth. Similarities between fossils and living species are best explained by evolution. Long periods of time, changes in the Earth’s surface and climate, and the similarities between fossils and living species were three of the observations that led Charles Darwin to propose the theory of evolution.
Offspring of organisms differ in small ways from their parents and generally from each other. This is called variation in a species.
Variation provides the raw material for natural selection to act upon. The variation is controlled by genes, and is transmitted from one generation to another through the genes.
Natural selection kills those individuals of a species which lack the characteristics that would have enabled them to survive and reproduce successfully in their environment. Individuals which have characteristics suited to the environment reproduce successfully and some of their offspring carry the successful characteristics. Natural selection is accelerated when the environment changes; this can lead to extinction of a species.
Natural selection is the mechanism whereby evolution results in changes within a species. Accumulated change in isolated populations eventually results in new species forming. The fact that characteristics are inherited through the genes means that individuals that reproduce successfully pass on their genes to the next generation. Species that do not adapt to an environmental change become extinct.
Variations in human biological characteristics such as skin colour, height, and so on have been used to categorise groups of people. These biological differences do not indicate differences in innate abilities of the groups concerned. Therefore, such categorisation of groups by biological differences is neither scientifically valid nor exact; it is a social construct.
Variations in physical characteristics are largely controlled by the genes. Changes in skin colour in humans result from a minor alteration in the genes, which is not associated with differences in intelligence or any other characteristic.
Extinctions also occur through natural events. Mass extinctions have occurred in the past, suggesting that huge changes to environments have occurred. However, these changes occurred very slowly, compared to the fast rate at which humans can destroy plant and animal species.
Extinction means that life on Earth is not the same as it was thousands, millions or hundreds of millions of years ago. The fossil record provides evidence of species that have existed on Earth in the past, and are no longer present on the Earth. The fossil record also provides evidence of periods of relatively rapid turnover in species, which are associated with massive environmental change, such as that caused by a meteorite impact.
Unifying statement: The Earth is composed of materials which are continually being changed by forces on and under the surface.
The Earth is constantly undergoing change: it is not the same as it was thousands or millions of years ago. Through realising that the surface of the Earth changes, Charles Darwin began to understand that life forms could evolve in response to the changing surface of the Earth.
Fossils are the remains of life forms that have been preserved in stone. Fossils are evidence that life, climates and environments in the past were very different from those of today.
Fossils provide tangible evidence of life forms that have existed in the past, and through which we can trace the evolutionary history of the species living on Earth at present. Fossils therefore provide strong evidence that life has evolved.
Many of the organisms in South Africa’s fossil record cannot be easily classified into groups of organisms alive today, and some are found in places where presentday conditions would not be suitable for them. This is evidence that life and conditions on the surface of Earth have changed through time.
Fossils provide direct evidence of differences and similarities in life-forms presently living on Earth. They also support the idea that evolution has resulted in changes in living organisms in response to changes in the Earth’s environment over very long periods of time.
Planet Earth and Beyond: The changing Earth
Table 2 shows that a large number of topics have direct relationships with evolution. The Natural Sciences learning area provides a number of the foundational concepts on which the theory of evolution was built. An understanding of genetics provides us with a deeper understanding of the actual mechanisms involved in the process of evolution.
Crossing Over: The Basics of Evolution
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HSRC Workbook
Table 2: Content topics in the Natural Sciences learning area statement and their relationship to evolution
Thus, evolution underpins the Natural Sciences learning area, although it is never named in the document. In South Africa we are very lucky as we have many unique attributes that make it easy to teach the learners about evolution in practical and meaningful ways. l Within our country’s borders, we have at least seven hotspots of biodiversity, where we can see the results of rapid evolution in particular areas. l We have a rich fossil record that extends from some of the oldest fossil bacteria in the world (3 600 million years old) to human fossils of the last 100 000 years. l We have a network of museums where learners can see and touch fossils, as well as world heritage site, the Cradle of Humankind, where learners can see evidence of the evolution of humans. l Through the work of geneticists in South Africa, we can draw on DNA analysis to understand human history extending back to 100 000 years ago and beyond.
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The Natural Sciences curriculum
HSRC Workbook
Using the workbook This workbook is written in the form of an interactive text so you can think and make notes in the book as you work through it. The activities are designed to develop your own understanding, but you may find that some activities are suitable for use with learners. Feel free to adapt the activities to suit your learners, but remember that some activities may be too difficult for school use. You may also translate portions of the text into the language of learning and teaching at your school. The learning outcomes, assessment standards and content topics covered in each section are listed at the beginning of the section to help you to structure your learning programme for Natural Sciences. Most of the material on genes and chromosomes is not included in the prescribed content for the Natural Sciences learning area, but it is important for your own understanding of the relationship between genetics and evolution. If you also teach Life Sciences, you may find sections of this workbook useful in your teaching in Grades 10 – 12. This workbook is available as a downloadable .pdf document from the HSRC website (www.hsrcpress.ac.za) and you may print pages to make your own audio-visual aids or worksheets for your learners. When we trialled the workbook, we found that the practical activities helped participants to enjoy and understand the concepts. In particular, allowing learners to touch and see real fossils helps them to understand that life existed millions of years ago, and that the life-forms were different from the life we see around us now. When you teach this material, try to arrange for learners to visit a museum, or borrow some fossils from the nearest museum so that learners get first-hand experience of fossils. You can buy an inexpensive cast of the skull of Mrs Ples, a famous pre-human fossil, from the Transvaal Museum. This book is only an introduction to some of the basic ideas of genes and inheritance. At the end of Part 2, you will find a list of resources that you can use to help you find out more about evolution. As more information becomes available each year, it is a good idea to search the Internet or your local library for new books and videos on the topic of evolution. New genetic discoveries are often reported in newspapers and magazines such as African Geographical and local newspapers.
Crossing Over: The Basics of Evolution
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HSRC Workbook
Understanding Genes & Inheritance
Part
1
Contents Overview
3
1. The cell
4
The structure of a cell
5
2. Measuring very small structures
9
Using scale bars
10
14
Cells divide Events in the cell cycle
14 15
4. The chromosomes and cell development
16
5. Mitosis Steps in the process Cell division
3. The cell cycle
19
19 21
23
23 24 25 26
28 28
32
32 34 34 34
6. Inheritance Parents and offspring What is a species? Chromosomes carry genes The genetic basis of inheritance The relationship between chromosomes and the whole organism Measuring variation 7. Selection Natural selection Natural selection and evolution Artifi cial selection Artifi cial selection and genetics
8. General principles of reproduction Reproduction ensures the survival of a species
36 36
9. Sexual reproduction Males and females Meiosis Fertilisation
38
38 40 42
Solutions to activities
44
1
HSRC Workbook
Overview Part 1 of this workbook explores the way every cell carries a plan for its development and functioning. The plan is stored in a coded form in the chromosomes. You will learn how this plan is copied and passed from cell to cell and from parent to offspring. Humans use the knowledge about these plans to breed animals and plants that are useful to us.
Skills Interpreting micrographs, converting measurements, carrying out a survey, drawing and interpreting frequency distribution graphs.
Outcomes When you have finished Part 1, you should be able to: l Identify structures in cells l Describe the cell cycle in drawings and words l Explain the role of chromosomes and genes in transferring hereditary information from cell to cell, and from parents to offspring l Draw and interpret a frequency distribution bar graph l Explain the uses of genetic control of development in tissue culture, cloning, plant and animal breeding l Describe the role of natural selection in evolution.
Crossing Over: The Basics of Evolution
1
The cell
Learning Outcome 2: Constructing science knowledge Assessment Standards Recalling meaningful information when needed Applying knowledge to problems that are not taught explicitly Learning Outcome 1: Scientific investigation Assessment Standards Conducting investigations and collecting data Evaluating data and communicating findings Knowledge area: Life and Living Substrand: Biodiversity, change and continuity Content topic: The cell is the basic unit of most living things, and an organism may be formed from one or many cells. Cells themselves carry on life processes such as nutrition, respiration, excretion and reproduction, which sustain the life of the organism as a whole.
Cells are the smallest units of life that can grow, reproduce and carry out metabolic functions. Most cells are too small to see with the naked eye. Your body consists of millions of cells, but your eyes are not powerful enough to distinguish even one cell without a microscope. Cells were first discovered in 1665 by a scientist called Robert Hooke. He sliced a piece of cork into a thin sheet. He then studied the thin sheet of cork using a homemade microscope. (A microscope makes things look much larger than their usual size). Hooke discovered that cork was made up of tiny boxes. He decided to call these boxes cells.
Figure 1. What Robert Hooke saw through his microscope.
Other scientists observed many living things under microscopes and came to the conclusion that all living things are made up of cells. Some organisms consist of only one cell. We say they are unicellular organisms. Organisms that are made up of many cells are called multicellular organisms.
4
Part 1: Understanding Genes and Inheritance
HSRC Workbook
The structure of a cell Cells come in many shapes and sizes. When we talk about a ‘typical’ cell, we are really talking about the characteristics that are shared by most cells. You can think of a cell as a tiny bag which holds water, thousands of very tiny molecules, and some other microscopic structures.
Q
Why do all living things consist of cells?
A
Cells carry out chemical reactions as part of their metabolism. By keeping the molecules in a restricted area, the reactions can take place much more efficiently. Scientists think cells evolved because it is more efficient for metabolic reactions to take place in a restricted environment than if the molecules were floating freely in a large amount of water.
Did you know? South African scientists have discovered fossils of cells in rocks that are about 3.5 billion years old. Life has existed on the Earth for more than 3.5 billion years!
cell wall
Mitochondrion cell membrane Nucleus Endoplasmic reticulum Chloroplast Vacuole Cytoplasm
Figure 2. The structure of a typical plant cell.
The cell membrane Earlier you read that you can think of a cell as a tiny bag. We call the bag that holds the cell contents the cell membrane. The cell membrane is a layer that separates the cell from the cells around it, or from the water or air that surrounds the cell. It is like a very thin skin surrounding the cell contents.
Crossing Over: The Basics of Evolution
Diffusion is the process whereby a substance moves from a region of high concentration to a region of lower concentration.
The cell membrane plays a very important role in the metabolism of a cell. It keeps all the molecules that take part in chemical reactions inside the cell. It allows small molecules like oxygen, carbon dioxide, water and some salts to diffuse freely into and out of the cell. The cell membrane helps certain molecules like sugars to move into or out of the cell. We call the process whereby a cell membrane helps molecules to move into or out of the cell active transport. Diffuse (say dife-fuse) means that a substance moves from where it is plentiful to where it is scarce. So, if carbon dioxide is plentiful in a cell, and scarce outside the cell, carbon dioxide will diffuse out of the cell. It diffuses through the cell membrane. If oxygen is plentiful inside a cell and scarce outside the cell, oxygen will diffuse out of the cell. It diffuses through the cell membrane. Animal cells use oxygen during the metabolic process called cellular respiration. Oxygen is scarce inside the cell, but plentiful outside the cell. Oxygen diffuses into each cell through the cell membrane. Cellular respiration produces carbon dioxide inside the cell. Carbon dioxide is plentiful inside the cell and scarce outside the cell. Carbon dioxide diffuses out of each cell through the cell membrane.
Activity 1 Imagine a cell in the leaf of a green plant. Underline the word or words that correctly complete these sentences about a cell during the daytime. During daylight, green plant cells make sugar by the metabolic process of (cellular respiration / photosynthesis). The process uses the gas (oxygen / carbon dioxide), which enters the cell by (active transport / diffusion) through the cell membrane. During the daytime, green plant cells produce the gas (oxygen / carbon dioxide). It leaves the cells by (diffusion / active transport) through the cell membrane. Photosynthesis produces molecules of (sugar / protein) in the leaves. Root cells break down (protein / sugar) in the metabolic process of (cellular respiration / photosynthesis). Sugar must move from the cells of the leaf to the cells of the root. Sugar cannot (diffuse / be actively transported) through the cell membrane. The cell membrane assists sugar to (enter / leave) the cells of the leaf and to (enter/ leave) the cells of the root.
Cytoplasm
Cytoplasm is the name given to everything inside a cell except the nucleus.
The substances and structures inside the cell membrane are called cytoplasm (say sy-toe-plaz-im). Cytoplasm consists of cytosol (sy-toe-sol) and organelles (say organ-els). The cytosol contains water, dissolved substances like sugar, salts, oxygen and carbon dioxide, and molecules that take part in the metabolic processes of the cell. The organelles are microscopic structures floating in the cytosol.
Part 1: Understanding Genes and Inheritance
HSRC Workbook
Activity 2 Look at Figure 2. Find the following organelles in the cytoplasm: a. Chloroplasts (say klaw-ro-plasts) b. Mitochondria (say my-toe-kon-dree-a) c. Endoplasmic reticulum (say en-doe-plas-mic re-tic-you-lum) d. Vacuole (say vac-you-ole)
Each organelle is separated from the cytoplasm by its own membrane. Some organelles have folded membranes inside an outer membrane. The whole cytoplasm contains membranes that are folded and branched inside the cell. Each organelle has a particular function in the cell. l Photosynthesis takes place in the chloroplasts. l Cellular respiration takes place in the mitochondria. l Proteins are manufactured in the endoplasmic reticulum. l Water, small molecules, and waste products are stored in the vacuole.
Organelles are microscopic structures in a cell.
Q
Mitochondria occur in both animal and plant cells, but only plant cells contain chloroplasts. Why is this?
A
Only plant cells carry out photosynthesis, which takes place in the chloroplasts. Therefore, we expect that only plant cells will have chloroplasts. The reactions of cellular respiration take place in the mitochondria. Both animal and plant cells carry out cellular respiration, therefore we expect that both plant and animal cells will contain mitochondria.
Activity 3 Would you expect to find chloroplasts in the cells of a plant root? Explain your answer.
Crossing Over: The Basics of Evolution
The nucleus In Figure 2 you saw that the nucleus is the biggest structure in the cell. When cells were first seen, the microscopes were powerful enough to enable scientists to see that a cell contained a nucleus and cytoplasm, but they could not see all the organelles that we now recognise. We use the term cytoplasm to mean everything inside the cell except the nucleus. Cytoplasm
Chromosomes
Figure 3. Chromosomes in a cell.
Chromosomes are structures in the nucleus that carry hereditary information.
8
The nucleus is separated from the cytoplasm by a membrane which has a large number of tiny holes, or pores, in it. The nucleus contains very important structures called chromosomes (say kroam-o-somes). A chromosome is a structure in the nucleus that carries the hereditary information for the cell. Hereditary information is information that passes from one cell or organism to its offspring. It is the set of instructions that control the way a cell develops and functions. Normally, we cannot see the chromosomes in a nucleus, but when a cell is about to divide, we see the chromosomes as threads in the cell. You can see a photograph of chromosomes in a cell in Figure 3. A chromosome is made up of a very long molecule called deoxyribonucleic acid (DNA) combined with proteins. The chromosomes carry all the instructions for the growth, reproduction and metabolism of the cell and for the whole organism.
Part 1: Understanding Genes and Inheritance
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2 M easuring very small structures Learning Outcome 2: Constructing science knowledge Assessment Standards Recalling meaningful information when needed Applying knowledge to problems that are not taught explicitly Learning Outcome 1: Scientific investigation Assessment Standards Conducting investigations and collecting data Evaluating data and communicating findings Knowledge area: Life and Living Substrand: Biodiversity, change and continuity Content topic: The cell is the basic unit of most living things, and an organism may be formed from one or many cells. Cells themselves carry on life processes such as nutrition, respiration, excretion and reproduction, which sustain the life of the organism as a whole.
The smallest division on most rulers is one millimetre, but cells are much smaller than one millimetre. You could fit ten human egg cells into one millimetre. Scientists use a special set of units to measure very small structures. These units and their relationship to one metre are shown in Table 1.
Units
Millimetre (mm)
Micrometre (μ)
Number of units in one metre
1 000
1 000 000
Units expressed in scientific
1 x 10
Nanometre (nm) 1 000 000 000
1 x 10
-3
-6
1 x 10 -9
notation as fraction of (m)
Scientific notation is a special way of representing very large or very small quantities in science. You can convert numbers from normal form to scientific notation easily: 1 000 = 10 x 10 x 10 = 1 x 103
Q A
There are three zeroes in 1 000. To what power is ten raised in the scientific notation for 1 000?
Table 1: Units for measuring small structures
Did you know? In mathematics, division is the inverse, or opposite of multiplication, so when we are expressing numbers as a fraction of another, the index, or power is negative. A micrometre is onethousandth of a millimetre, so we say it is 1 x 10–3 mm, or 1 x 10–6 m.
Three.
It is difficult for us to picture such tiny amounts, but try to imagine that one millimetre division on your ruler divided into one thousand segments. Each segment would be one micrometre.
Crossing Over: The Basics of Evolution
9
Using scientific notation, we can say that one micrometre is 1 x 10 -3 mm. Micrometres are also called microns. If one micrometre was divided into one thousand equal segments, each segment would be one nanometre. Using scientific notation, we say that one nanometre is 1 x 10 -6 mm.
Using scale bars Biological drawings often have a scale bar on the diagram which gives you an idea of the actual size of the specimen.
A
0.4 mm
Figure 4. Transverse section of a leaf.
Scale bars are used to work out the magnification of the drawing and the actual size of the specimen. The magnification on the bar tells us how much bigger or smaller than the real specimen the drawing or micrograph is. For example, the scale bar on Figure 4 shows how long 400 µm would be at the magnification of the drawing. How much bigger or smaller is the drawing than the real specimen? 1. Measure the length of the scale bar. It is 20 mm. 2. 20 mm represents 400 µm of the actual specimen. Convert the 20 mm to µm by multiplying by 1 000. 3. 20 x 1 000 = 20 000 µm. 4. Now divide the actual measurement of the scale bar by the measurement that it represents. 5. 20 000 ÷ 400 = 50. The diagram shows the actual object magnified 50 times. We write the magnification like this: 50x or x50 You can also use the scale bar to measure the actual size of various parts of the diagram. For example, let’s say you want to know how tall the cell marked A is. 1. Measure the height of cell A with your ruler. It is 10 mm high.
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2. According to the scale of the diagram, its magnification is 50x. Cell A is drawn 50 times bigger than its real size. To calculate its real size, divide 10 mm by 50. 3. The answer is 0,2 mm or 200 µm. Cell A is part of a layer of similar cells, but the cells are not all the same size. Measuring only one cell is not a true reflection of all the cells in that layer. In order to make a general statement about the height of cells in layer A, you should measure a number of cells and calculate the average. For example, measure five cells. 10 mm, 9 mm, 9,5 mm, 11 mm, 10 mm. To calculate the average height, total all the measurements and divide by the number of measurements. 10 + 9 + 9,5 + 11 + 10 = 49,5 = 9,9 mm 5 Convert the average height in the diagram to the actual height using the magnification (x50), in other words, divide by 50. 9,9 = 0,198 mm, or 198 µm. 50
Activity 4 1. Figure 5 shows some cells from the inside of a human’s mouth. a. Use the scale bar to work out the actual diameter of five cells. b. Work out the average diameter of the cells.
Figure 5. Human cheek cells with scale bar.
Crossing Over: The Basics of Evolution
11
2. Figure 6 shows some cells from the surface of a leaf. a. Use the scale bar to work out the actual length and width of five cells. b. Work out the average length and width of a cell from the surface of a leaf.
Figure 6. Epidermal cells with scale bar.
3. Figure 7 shows some bacterial cells. Use the scale bar to work out the average length and width of the round bacterial cells.
Figure 7. S canning Electron Micrograph of some bacteria from the gut of a nyala.
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HSRC Workbook
4. You have now measured three different kinds of cells: plant cells, animal cells and bacterial cells. Compare the sizes of the three kinds of cells you have measured. What do you notice?
5. Draw lines to match each cell structure with its function: Cell membrane
carries out photosynthesis.
Nucleus
carries the hereditary information of the cell.
Vacuole
carries out cellular respiration.
Mitochondria
manufactures proteins.
Chloroplast
stores water, small molecules and waste products.
Endoplasmic reticulum
allows substances to enter and leave the cell
Crossing Over: The Basics of Evolution
13
3 The cell cycle Learning Outcome 2: Constructing science knowledge Assessment Standards Recalling meaningful information when needed Applying knowledge to problems that are not taught explicitly Categorising information to reduce complexity and look for patterns Interpreting information Knowledge area: Life and Living Substrand: Biodiversity, change and continuity Content topic: The cell is the basic unit of most living things, and an organism may be formed from one or many cells. Cells themselves carry on life processes such as nutrition, respiration, excretion and reproduction, which sustain the life of the organism as a whole.
It was clear in Unit 1 that the smallest unit of life is a cell. Life cannot be created from non-living materials, even under the most carefully controlled conditions. No-one has managed to create a living cell by mixing together various chemicals and supplying energy. Since cells cannot be created, where do new cells come from? Your body, consisting of millions and millions of cells, grew from a single fertilised egg. A tree grows from a single fertilised egg cell in the flower. Where do all the extra cells come from? Cell theory says two things about cells: l All living organisms are made up of cells. l All cells arise from other cells. How does one fertilised egg cell grow into millions and millions of cells in a human body?
Cells divide The first fertilised egg cell of any organism grows into many cells by dividing. We say cells pass through a cell cycle. A cell grows to its full size, and then divides into two daughter cells. Each daughter cell grows to its full size, and then divides into two daughter cells. The cell cycle is the time from the formation of a daughter cell to its own division into two daughter cells. The cell cycle is illustrated in Figure 8. ild egd\Zcn XZaah
14
Y^k^h^dc \gdli]
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Figure 8. A sequence of cell divisions. The letter A marks the start of a cell cycle, and B marks the end of a cell cycle.
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6
7 \gdli]
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Part 1: Understanding Genes and Inheritance
HSRC Workbook
Notice that when a cell divides into two daughter cells, the original cell effectively disappears. All cells in a developing embryo go through repeated cell cycles, but once cells have become specialised for a particular function, they no longer divide. Cell division only takes place in certain specialised parts of the adult body. For example, cells under the surface of your skin divide continuously to replace cells that are worn off on the outside of your skin. The duration of a cell cycle varies from a few hours to several weeks. The cells at the tip of many roots have a cell cycle of about 12 hours, while cells in human skin have a cell cycle of about 12 hours. Bacterial cells have a cell cycle of 20 minutes under very good conditions.
Events in the cell cycle Growth I A newly-formed daughter cell is about half the size of an adult cell, so the first phase of the cell cycle is taken up with growing. The cell makes new organelles, more cytosol is produced, and the cell wall increases in size. The growth phase is the longest part of the cell cycle, and the most variable. Some cells never progress beyond the growth phase, others become specialised for particular functions and stay in the growth phase until they die.
Replication (say rep-li-cay-shun) Each chromosome in the nucleus makes an identical copy of itself in the replication phase. At the end of the replication phase, the cell contains a double set of chromosomes.
Growth II The cell grows a little bigger. The second growth phase is shorter than the first growth phase.
i] h
]
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gh
jg
dj
\gdli]>
Cell division
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gZea^XVi^dc e]VhZ
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The first part of cell division is called mitosis (say my-toe-sis). We will describe mitosis in more detail in section 5. During mitosis, the two sets of chromosomes produced in the replication phase separate and move to opposite sides of the cell. In the second part of cell division, a new cell membrane forms across the middle of the parent cell, dividing it into two daughter cells. The organelles of the parent cell are shared between the two daughter cells.
dc
h i
d
b
Crossing Over: The Basics of Evolution
Figure 9. The cell cycle. The dark shaded area is called interphase.
15
4 T he chromosomes and cell development This section contains some advanced knowledge and exercises that may not be suitable for all Senior Phase classes. Use your discretion, or adapt the activities to suit your class. Learning Outcome 2: Constructing science knowledge Assessment Standards Recalling meaningful information when needed Applying knowledge to problems that are not taught explicitly Categorising information to reduce complexity and look for patterns Interpreting information Knowledge area: Life and Living Substrand: Biodiversity, change and continuity. Content topic: The cell is the basic unit of most living things, and an organism may be formed from one or many cells. Cells themselves carry on life processes such as nutrition, respiration, excretion and reproduction, which sustain the life of the organism as a whole.
Activity 5 1. Where do you find chromosomes in a cell?
2. What are chromosomes made of?
3. What is the important function that chromosomes carry out in a cell?
4. Think about all the metabolic functions that take place in a cell. What controls the metabolic functions?
5. Think about the fact that every human being develops from a single fertilised egg cell. What controls the way an embryo develops?
6. What makes sure that arms grow in the right places, legs grow in the right places, and all the organs form in the right places?
7. What makes sure that the embryo grows into another human being, and not into another kind of organism?
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Part 1: Understanding Genes and Inheritance
HSRC Workbook
The answer to all these questions is that the chromosomes carry all the information that a cell needs to grow and to carry out its functions. The chromosomes direct each cell in the path of its development. As each cell develops in response to the instructions from the chromosomes, it forms part of a structure in the embryo. You should remember from section 1 that each chromosome consists of a long molecule of DNA, combined with protein. Chromosomes carry the hereditary information, that is, the information that passes from one cell to the next and from an adult organism to its offspring. Hereditary information is vital for a cell to function, and for an organism to develop and function correctly.
Figure 10. A human baby grows from a single fertilised egg cell.
Some cells become bone cells, some cells become muscle cells, some become nerve cells, some become skin cells, and some cells become blood cells. Almost every single cell in an organism has a full set of chromosomes for the whole organism. We say ‘almost’, because a few specialised cells, like red blood cells in mammals, and phloem cells in plants, do not have chromosomes. Chromosomes pass from parent cell to daughter cell. Normally, we cannot see the chromosomes in a cell, but when a cell is about to divide, the chromosomes become visible under a microscope.
Crossing Over: The Basics of Evolution
17
Look at Figure 11, which shows the cells in the root tip of an onion. You can see that some cells have a large, dark-stained nucleus, while others have small threads in the cell. The threads that you can see in some cells are the chromosomes. D
Figure 11. We can only see chromosomes when cells are dividing. A
B
C
E
Scientists use special techniques to spread out the chromosomes of a cell. They have found that all the cells of a particular kind of organism have the same number of chromosomes. For example, each cell in a human body has 46 chromosomes; each cell in the body of a fruit-fly has 4 chromosomes, and each cell in a maize plant has twenty chromosomes. Before a cell divides, each chromosome makes an exact copy of itself during the replication phase of the cell cycle. A cell in a human body after the replication phase has twice as many chromosomes as a normal cell: it has 92 chromosomes. A fruit-fly cell has eight chromosomes after replication, and a maize cell has 40 chromosomes after replication. The chromosomes in a cell that is about to divide are arranged in pairs, which we call chromatids (say kroa-ma-tids). At this stage in a cell cycle, each chromosome consists of two identical chromatids, joined together by a structure called the centromere (say sen-tro-meer)
Activity 6 Match each term with its meaning:
18
Chromosome
The place where two chromatids are attached.
Spindle
The process of producing new individuals without fertilisation.
Chromatid
The threads that stretch across a cell that is about to divide.
Centromere
A structure in the nucleus that carries hereditary information.
Cloning
One half of a chromosome after replication.
Part 1: Understanding Genes and Inheritance
HSRC Workbook
5 Mitosis
This section contains some advanced knowledge and exercises that may not be suitable for all Senior Phase classes. Use your discretion, or adapt the activities to suit your class. Learning Outcome 2: Constructing science knowledge Assessment Standards Recalling meaningful information when needed Applying knowledge to problems that are not taught explicitly Categorising information to reduce complexity and look for patterns Interpreting information Knowledge area: Life and Living Substrand: Biodiversity, change and continuity Content topic: The cell is the basic unit of most living things, and an organism may be formed from one or many cells. Cells themselves carry on life processes such as nutrition, respiration, excretion and reproduction, which sustain the life of the organism as a whole.
Steps in the process Prophase At the beginning of mitosis, the chromosomes coil tightly, like a spring, so that they become thicker and fatter. At this stage, each chromosome consists of two chromatids, joined at the centromere. The membrane that normally surrounds the nucleus now disappears, so the chromosomes float in the cytosol.
Metaphase Fine threads form in the cytosol, making a pattern called a spindle in the cell. The chromosomes attach themselves to the spindle at the centromere. The spindle threads become tight and begin to pull the chromosomes, so that they all line up in the middle of the spindle.
Anaphase The two chromatids of each chromosome suddenly pull apart and move towards opposite sides of the cell.
Telophase The threads of the spindle disappear. The chromosomes begin to uncoil and cluster together to form two new nuclei. A new nuclear membrane forms around each nucleus. Mitosis is now complete, but before the cell cycle has ended, the cytoplasm must divide into two.
Crossing Over: The Basics of Evolution
19
Chromosomes condensing. Each chromosome has already divided into two chromatids
Cell membrane Spindle
Chromosomes arranged in an orderly manner about the plane of the spindle
Chromosomes moving to the equator of the spindle in a disorderly manner
Spindle
Aster
Centriole
Figure 12. Mitosis in animal cells – micrographs.
20
Figure 12. Mitosis in animal cells – drawings.
Part 1: Understanding Genes and Inheritance
Cell membrane
Spindle fibres. Each daughter centromere has a spindle fibre attached to it. Contraction of the fibre pulls the daughter centromeres to opposite poles
Cleavage furrow grows inwards
Chromosomes at poles. Nuclear membranes enclose the chromosomes at each pole
Figure 12. Mitosis in animal cells – micrographs.
Figure 12. Mitosis in animal cells – drawings.
Cell division A new cell membrane starts to form between the two nuclei. Once the new membrane has divided the cytoplasm into two new cells, cell division is complete. The two new cells begin another cell cycle.
Q
Each new cell has the same number of chromosomes as the original cell. Why is this important?
A
Each cell in an organism must have a complete set of chromosomes to ensure that all the functions of the cell can be carried out. If the cell did not replicate its chromosomes before it divides, some daughter cells may be missing essential chromosomes and die.
Crossing Over: The Basics of Evolution
21
HSRC Workbook
One of each pair of chromatids moves to each pole. Once they are separated, the chromatids are called chromosomes
Activity 7 1. Figure 11 is a micrograph of cells in the tip of a root. You can see different phases of mitosis in some cells. Certain cells have been labelled A, B, C, D, and E. a. What phase of mitosis is each labelled cell in? A. B. C. D. E. b. Draw each cell and label the chromosomes, spindle, nucleus and cell wall wherever you can identify those structures.
2. Suggest a simple, practical way that you could demonstrate mitosis to a Grade 9 class. You could use pieces of wool or string to represent the chromosomes, and a sheet of paper to represent the cell. Demonstrate your model of mitosis to your colleagues. 3. Draw a diagram representing the first five cell cycles after an egg has been fertilised. Assume that every cell divides in each cell cycle. How many cells are present in the embryo after five cell cycles?
4. A fertilised egg cell in a maize plant has twenty chromosomes. a. How many chromosomes are present in each cell of the adult maize plant? b. How many chromatids are present in a cell that is about to divide? c. How many chromosomes are present in each cell after cell division? 5. Before mitosis begins, each chromosome makes an exact copy of itself. Why is replication such an important event in the cell cycle?
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Part 1: Understanding Genes and Inheritance
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6 Inheritance
Learning outcome 2: Constructing science knowledge Assessment Standards Applying knowledge to problems that are not taught explicitly Interpreting information Recalling meaningful information when needed Learning outcome 1: Scientific investigation Assessment Standard Conducting investigations and collecting data Knowledge area: Life and Living Substrand: Biodiversity, change and continuity Content topics: Offspring of organisms differ in small ways from their parents and generally from each other. This is called variation in a species. Sexual reproduction is the process by which two individual plants or animals produce another generation of individuals. The next generation’s individuals look like the parents but always have slight differences (‘variation’) from their parents and from each other.
To inherit something means to receive it from a previous generation. You can inherit money from a late parent if he or she left it to you in their will. You can also inherit physical characteristics from your parents. For example, children can inherit certain diseases from their parents. Children also inherit physical characteristics from their parents. Usually, the children of tall parents will also be tall, and the children of short parents will be short. The children inherit the physical characteristic of their height from their parents. This section explores the biological mechanisms of inheritance.
Parents and offspring All living organisms inherit characteristics from their parents. It sounds strange to talk about the ‘parents’ of a plant or a fungus. In a biological sense, the male parent of any organism is the individual that supplied the sperm. The female parent is the individual that supplied the egg. So, in a maize plant … l The male parent is the plant that supplied the pollen which contains the sperm cells. l The female parent is the plant that supplied the cob, containing egg cells. Most organisms produced by sexual reproduction have two parents: a male parent and a female parent. We refer to the products of fertilisation as the ‘offspring’ of two individuals. Your own children are your offspring, and you are the offspring of your parents.
Similar, identical and variable In this section we will use words like resemble, looks like, similar, the same as, identical, varies, different, exactly and the same as. It is important that you know what these words mean.
Crossing Over: The Basics of Evolution
23
Resemble, looks like and similar are words that mean that the objects share certain characteristics. For example, look at the herd of cows in Figure 14. The cows share certain characteristics: they all have four legs, a body, a head, a neck and a tail. They all have horns and their skin is covered with fur. We say the cows resemble each other, they look alike, or they are similar to each other.
Figure 14. Cows and goats.
Identical and exactly the same as are words that mean that two or more objects share all their characteristics. Look at the herd of cows in Figure 14, notice that most of the cows are similar to each other, but they are not identical. Find two cows that have exactly the same pattern of markings and the same shape of their horns. We say these two cows are identical. They are exactly the same as each other. Varies and different are words that mean that objects differ in certain characteristics. For example, in Figure 14 you can see that cows are different from goats. Cows are bigger than goats, for one thing. Cows and goats make different sounds. When you look at all the cows, you can see that most of the cows differ from each other in their coat patterns. We say that their coat pattern varies.
What is a species? One of the fundamental characteristics of life is that particular kinds of organisms always produce offspring that resemble themselves. A group of individuals that breed and produce offspring that resemble themselves is called a species (say speeseez). A species is a group of organisms that resemble each other, and that breeds and produce offspring that resemble themselves. Here are some examples of offspring that grow up to resemble their parents: l Baby monkeys grow to look like monkeys. l Maize seeds grow into maize plants.
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Part 1: Understanding Genes and Inheritance
l l
HSRC Workbook
l
Hen eggs hatch into chickens that grow into domestic fowls. Duck eggs hatch into ducklings that grow into adult ducks. Thorn tree seeds grow into thorn trees.
In all the examples of species above, the young grow from a single fertilised egg. One sperm cell from the father fertilises one egg cell from the mother, and the offspring grows from that one fertilised cell. The offspring inherit something from the parents that makes sure they grow into the same species as the parents. Whatever it is that passes to the offspring is carried in the sperm cell and the egg cell, so it must be very small but very powerful. It must carry all the instructions to make sure that the fertilised egg cell grows into a whole new organism that resembles the parents. The structure that offspring inherit from the parents is a set of chromosomes. Each species has a set of chromosomes that ensure that all individuals of that species will look alike. Each fertilised egg must have a complete set of chromosomes for the species.
Figure 15. One sperm fertilises an egg cell.
Chromosomes carry genes In section 5, you learnt that chromosomes carry all the instructions that a cell needs to grow and carry out its functions. Chromosomes consist of long molecules of DNA, wound around protein molecules.
Q
A
What kind of instructions does a chromosome carry?
The chromosomes carry codes for the sequences of amino acids in particular proteins.
Q
How do proteins control the way a cell grows, develops and functions? How can proteins make different colours in skin, and different shapes to leaves?
A
Cells make different kinds of proteins. Most of the proteins that control the way a cell grows, develops and functions are enzymes. Enzymes speed up chemical reactions in cells. The chemical reactions build new substances, break down old ones, and enable cells to do different things.
A gene is a section of a chromosome that carries the code for a particular protein, and therefore controls a particular characteristic or process. Some proteins are not enzymes, but they form structures in cells. For example, skin cells produce a protein called collagen that makes skin strong. Hair cells produce a protein called keratin that supports the hair. Each chromosome carries codes for several thousand protein molecules. The codes for a single protein molecule are arranged in a particular area of the chromosome. We call the section of a chromosome that carries codes for a particular protein a gene (say jean).
Crossing Over: The Basics of Evolution
25
Big sections of chromosomes do not code for any proteins. Most of the genes in a particular cell are never decoded. Imagine that the genes are switched off. But almost every cell in your body has all the genes needed to make your whole body grow and function correctly.
Q
How do we know when a gene is ‘switched on’?
A
Genes that are active swell up and form ‘puffs’. Scientists can see parts of the chromosomes where genes are active.
The genetic basis of inheritance The action of genes is to produce proteins that gradually build a structure, a substance or break down structures in a cell. Each gene controls one protein, but it takes many protein molecules to make a structure. We must assume that most physical features of a living organism are the result of the work of many proteins, and therefore many genes. Each cell in the human body has about 30 000 genes – that is the total number of genes that are needed to make a whole human being from a single fertilised egg cell. Identical genes produce identical proteins, which make identical structures. So, if two individuals are identical in some genetically-controlled characteristic, we assume that their genes are identical. The opposite of this is that if two individuals differ in the appearance of a genetically-controlled characteristic, we assume that their genes differ. All members of a species share a number of important characteristics, so we must assume that thousands of genes for each species are identical. However, individuals within a species vary in a number of characteristics. For example, in Figure 14, you noticed that cows vary in the patterns of their coats.
Activity 8 1. Write down three ways that cows vary in their physical characteristics.
2. Write down three ways that humans vary in their physical characteristics.
3. Think of your own family. Do all the children of one mother and father look exactly the same?
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Part 1: Understanding Genes and Inheritance
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Even very closely related members of the same species vary in detail. You have probably noticed that children of one mother and father often resemble each other. Sometimes we can tell who a child’s parents are by looking at the child!
You have to be a Robertson! You have the Robertson eyes and nose.
I can see that you are Themba Mtshali’s brother – you look just like him.
You are surely from the Khan family. They are all short and thin like you.
You must be the daughter of Lindiwe Mbanjwa.
Figure 16. What physical characteristic have these children inherited from their father?
Activity 9 1. Nonhlanhla and Ntombifuthi are identical twins, while Zanele and Zodwa are sisters from the same mother and father. What can you say about the genes of Nonhlanhla and Ntombifuthi?
2. What can you say about the genes of Zanele and Zodwa?
Crossing Over: The Basics of Evolution
27
The relationship between chromosomes and the whole organism 8=GDBDHDB:H
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DC>C<D;I=: L=DA:DG<6C>HB Figure 17. The relationship between chromosomes and the whole organism can be summarised in a diagram like this one.
Measuring variation We can measure the variation in physical characteristics of a species in many ways. Here are some examples. Table 2 shows the masses of the cobs from one hundred maize plants of a particular variety. The maize plants were grown in one field, and they all received the same amount of fertiliser. The maize plants all belong to one species. Table 2: Masses of one hundred maize cobs
Mass (g)
<200
201 – 250
251 – 300
>300
Total
No. of cobs
16
25
46
13
100
Table 2 groups the maize cobs into four different mass ranges. Most of the maize cobs (46 out of 100) were in the mass range 251-300 g. A few cobs (13 out of 100) had a mass greater than 300 g. Only 16 out of 100 maize cobs had a mass of less than 200 g.
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Part 1: Understanding Genes and Inheritance
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Domestic hens vary in the number of eggs they lay each week. Mrs Mnisi keeps twenty hens in separate cages. Each hen receives the same amount of food and water each day. Mrs Mnisi counts the number of eggs she receives from each hen in one week. Her results are shown in Table 3. Table 3: Egg production by Mrs Mnisi’s hens
No. of eggs
2
3
4
5
6
7
No. of hens
2
1
7
6
3
1
Table 3 tells you that Mrs. Mnisi’s hens laid between two and seven eggs in a week. Two hens laid two eggs each, one hen laid three eggs, seven hens laid four eggs, six hens laid five eggs, three hens laid six eggs, and one hen laid seven eggs. Most of the hens (thirteen out of twenty) laid four or five eggs in the week.
Activity 10 1. The graph in Figure 18 shows the results of a survey of the heights of twelve-year old boys in a particular school. &'
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Figure 18. Heights of twelve-year-old boys.
a. What do the figures along the bottom of the graph tell you?
b. What do the figures on the left-hand side of the graph tell you?
Crossing Over: The Basics of Evolution
29
Height (cm)
155 – 157
c. Complete this sentence: Most of the twelve-year-old boys are between ____ cm and ____ cm tall. d. How many boys have genes for being more than 165 cm tall at the age of twelve years? ________ e. What factor/s other than genes could influence the height of boys at age twelve?
f. Find information on the graph to complete this table:
157 – 159
159 – 161
161 – 163
163 – 165
165 – 167
167 – 169
169 –171
Number of boys 2. Draw a graph for the data in Table 2. 3. Draw a graph for the data in Table 3. 4. The ability to roll the sides of the tongue is inherited in humans. Stick out your tongue and try to roll up the sides. Look at Figure 19 to see how to do it.
Figure 19. Some people can roll their tongues, others cannot.
Some people can’t roll the sides of their tongue. Do a survey in your school. Choose a single class, and count the number of children who can roll their tongues, and the number who cannot. In the general population, the ratio of tongue-rollers to non tongue-rollers is about 3:1. Do your findings fit with this?
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Part 1: Understanding Genes and Inheritance
HSRC Workbook
5. Sandile Zondi is a Zoology student who is observing a group of zebras for a research project. Sandile knows that zebras vary in the patterns of stripes in their coats. Sandile needs to identify individual zebras for the project. Look at the picture of Sandile’s group of zebras. Make a list of distinguishing marks on each zebra that will enable Sandile to identify each individual.
6. Use the information in this Unit to fill in the missing words in this text: The nucleus of a living ________ contains long, thin strands called ________. Each chromosome contains smaller parts called ________ . Genes control the development of inherited ________. For example, in humans, genes control the development of skin colour, eye colour, ________, the shape of the nose, ________ and the build of the person. A sperm contains chromosomes with a set of genes from________. An egg (ovum) contains chromosomes with a set of genes from ________. At ________ these two sets of genes are brought together. They control development of the fertilised ovum into ________. The ________ of two parents inherits genes from both ________. The combination of genes that each individual inherits is unique, which means that each individual of a species is slightly different from all other individuals of its species. The exception to this rule is ________ . They share all their genes with each other, and are identical in all ________ characteristics.
Crossing Over: The Basics of Evolution
31
7 Selection
Learning outcome 2: Constructing science knowledge Assessment standard Interpreting information Learning outcome 3: Science, society and the environment Assessment standard Understanding science and technology in the context of history and indigenous knowledge Knowledge area: Life and Living Substrand: Biodiversity, change and continuity. Content topics: Natural selection kills those individuals of a species which lack the characteristics that would have enabled them to survive and reproduce successfully in their environment. Individuals who have characteristics suited to their environment reproduce successfully and some of their offspring carry the successful characteristics. Natural selection is accelerated when the environment changes; this can lead to the extinction of species.
Because of variability in certain genes, all individuals of a species differ in certain characteristics. Activity 10 showed several examples of variability that is genetically controlled. Here are some more examples of variability within a species: l Flowers on different plants of the same species may be blue or white. l Plants may vary in their ability to survive a period of drought. l Birds of a particular species may vary in their resistance to disease. l Buck of a particular species vary in the length of their horns. It is important to remember that only characteristics that are controlled by genes can be passed on to the next generation. Any features of an organism that result from environmental causes are not passed on to the next generation. Variability means that some individuals have a better chance of breeding, and therefore passing on their genes to the next generation, than others. In the examples above: l White flowers attract bees more easily than blue flowers. More white flowers will be pollinated, and more seeds will be produced by the white flowers. In the population as a whole, white flowers will become more common than blue flowers. l Plants that are drought resistant will survive and produce more plants that are drought resistant, if resistance to drought is genetically controlled. l Birds that are resistant to a disease are more likely to reproduce than birds that are not resistant to the disease. l Buck that have longer horns may be able to fight off other males that have shorter horns. The males with longer horns will be able to mate with more females than buck with shorter horns.
Natural selection The examples above show how nature favours certain individuals in each generation, and those individuals have a better chance of reproducing than other individuals. Remember that reproduction is the only way that genes,
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Part 1: Understanding Genes and Inheritance
HSRC Workbook
and therefore physical characteristics, can be passed on from one generation to another. Nature selects the best physical characteristics, but since those are controlled by genes, nature is really selecting genes. In nature, many individuals never reproduce, or their seeds, eggs or young die before they mature. Think of all the seeds that an Acacia tree produces each year. How many seeds grow into new trees? In nature, as few as 0,01% of the seeds may grow into adult trees. Natural selection is a very powerful process that makes sure that the best genes pass on to the next generation, so most species are very well adapted to their environment. Any individual that cannot escape from its predators, find food, water and shelter, and that is not resistant to disease or parasites will die without reproducing.
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Figure 20. Natural selection leads to changes in the coat colours of mice.
Crossing Over: The Basics of Evolution
33
Natural selection and evolution If natural selection continues over a long period of time, a new species may form. The new species will not be able to breed with the old species from which it originated. Its genes have changed so much that any offspring will be sterile, or fertilisation will not take place. Natural selection is the mechanism whereby evolution takes place. The huge diversity of living organisms that we see in the world today has evolved from species that are now extinct. Evolution is the most important concept in Biology, because it helps us to understand the processes that result in the diversity of life around us, similarities and differences in plants and animals, and the way organisms are distributed on the continents around the world.
Artificial selection Imagine that you were alive 10 000 years ago. You would most likely have lived in a small band of people who hunted for their meat and collected plant bulbs, roots, and leaves to eat. Let’s assume that while hunting you found some goats that were easy to catch and kill for meat. The other goats were very wild and difficult to capture. You realise that life would be much easier if you could capture a few of the tame goats and keep them near your settlement. When you do that, you discover that some of the offspring of the tame goats are also tame. You kill the wilder offspring to eat them and keep the tame offspring to breed. After a number of generations of selecting the tame goats to breed in each generation, you have a flock of tame goats that stay near the humans. The early farmers did not know about genes, but they did know that certain characteristics were passed from one generation to the next. By selective breeding of plants and animals, humans produced new breeds of plants and animals to provide for their needs in terms of food, work, sport, shelter and hobbies. Artificial selection works in a similar way to natural selection, but humans make the choices about which animals or plants will breed in each generation. In natural selection, the natural environment determines which individuals are best adapted, and therefore most likely to breed. Through artificial selection, the world’s population has been able to expand to its present huge numbers. Artificial selection, resulting in increased productivity, has been one of the key reasons for the success of the human species.
Artificial selection and genetics Artificial selection began with mass selection. Breeders selected the animals or plants that showed the best characteristics for a particular purpose. The breeder collected seed only from the best plants, and allowed only the best animals to breed. Mass selection is slow and unpredictable, although it does eventually produce the desired results.
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Part 1: Understanding Genes and Inheritance
HSRC Workbook
After the discovery of genes in 1900, breeders were able to work out more accurately what the results of a particular breeding were likely to be. They could use mathematical models to predict how many offspring of a particular cross were likely to have the desired characteristic. Modern breeding methods produce much quicker and more reliable improvements in agriculturally useful organisms.
Summary l
l
l
l
l
l
l
l
Each
cell passes through a cell cycle, in which it grows, replicates its chromosomes and divides into two daughter cells. Chromosomes carry the hereditary material for the development and functioning of the whole organism. Each cell inherits a full set of chromosomes from the parent cell. Mitosis is a precise sequence of events that results in a cell dividing into two daughter cells. Each daughter cell receives a complete set of chromosomes, and daughter cells are genetically identical to each other and identical to the parent cell. The hereditary units on chromosomes are called genes. Each gene controls the production of a single protein, which contributes to the development of a particular characteristic in the organism. Organisms that are physically and physiologically identical are genetically identical. Organisms that are physically and physiologically different are genetically different. Each species has a common set of genes that ensure that all members of the species look alike. Each individual within a species also has some genes that are variable, and that result in all individuals being slightly different from each other. Natural selection ensures that the individuals in each generation that are best adapted to the environment are most likely to breed, and therefore pass on their genes to the next generation. Natural selection results in evolution. Artificial selection refers to human selection of individual plants or animals for breeding. Artificial selection is the basis of animal and plant breeding techniques.
Crossing Over: The Basics of Evolution
35
8 G eneral principles of reproduction
Learning outcome 2: Constructing science knowledge Assessment Standards Applying knowledge to problems that are not taught explicitly Interpreting information Recalling meaningful information when needed Learning outcome 1: Scientific investigation Assessment Standards Planning investigations Conducting investigations and collecting data Evaluating data and communicating findings Knowledge area: Life and Living Substrand: Biodiversity, change and continuity Content topic: Sexual reproduction is the process by which two individual plants or animals produce another generation of individuals. The next generation’s individuals look like the parents but always have slight differences (‘variation’) from their parents and from each other. Substrand: Life processes and healthy living Content topic: Human reproduction begins with the fusion of sex cells from mother and father, carrying the patterns for some characteristics of each.
Reproduction ensures the survival of a species Every individual organism of a species has a life cycle, which lasts from the time it is born until it dies. A life cycle varies from twenty minutes for some bacteria to thousands of years for some trees. All living things, even if they are single-celled bacteria, will die. Remember that all life comes from existing life. The only way that a new individual can be produced is if another individual of the same species reproduces. Reproduction ensures that each species of organism present on Earth continues to survive. If all the individuals of a species stop reproducing, the species becomes extinct. Reproduction ensures that hereditary information passes from one generation to the next. Remember that each cell of every organism contains nuclear material. The nuclear material carries the hereditary information - the coded instructions that control the way every organism functions and develops. During reproduction, the hereditary information passes from one generation (the parents) to a new generation (the offspring). The hereditary information ensures that the offspring will resemble the parents and each other. The offspring will belong to the same species as their parents.
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Part 1: Understanding Genes and Inheritance
HSRC Workbook
Organisms reproduce sexually and asexually Methods of reproduction can be divided into two categories: l In sexual reproduction, two specialised sex cells (gametes) join and grow into a new organism. An individual produced by sexual reproduction has two parents, a male parent and a female parent. l In asexual reproduction, one individual (the parent) produces offspring without the fusion of specialised sex cells. Cloning is a form of asexual reproduction. An individual produced by asexual reproduction has only one parent.
Activity 11 1. Explain why all individuals of a species that reproduces asexually are genetically identical to each other.
2. Explain why all individuals of a species that reproduces sexually are genetically different from each other.
3. Complete the table to summarise the differences between asexual and sexual reproduction. Asexual reproduction
Sexual reproduction Two parents
No special sex cells required All offspring are genetically identical to the parent and to each other Occurs in plants and animals
Crossing Over: The Basics of Evolution
37
9 Sexual reproduction
Learning outcome 2: Constructing science knowledge Assessment Standards Applying knowledge to problems that are not taught explicitly Interpreting information Recalling meaningful information when needed Learning outcome 1: Scientific investigation Assessment Standards Planning investigations Conducting investigations and collecting data Evaluating data and communicating findings Knowledge area: Life and Living Substrand: Biodiversity, change and continuity. Content topic: Sexual reproduction is the process by which two individual plants or animals produce another generation of individuals. The next generation’s individuals look like the parents but always have slight differences (‘variation’) from their parents and from each other. Substrand: Life processes and healthy living Content topic: Human reproduction begins with the fusion of sex cells from mother and father, carrying the patterns for some characteristics of each.
Organisms that reproduce sexually produce special cells for reproduction, called the sex cells or gametes. Gametes are usually produced in special parts of the plant or animal.
Males and females In asexual reproduction, there is only one parent, so we do not distinguish between males and females. However, sexual reproduction involves two parents that produce two different kinds of gametes. In sexual reproduction, male individuals or organs produce sperm that can swim or that move from their parent. Female individuals or organs produce much larger gametes that do not move. The large gametes are called ova (singular ovum). Ova contain stored food. Some individual plants or animals produce both sperm and ova, while in others the sexes are separate.
Q A
38
We can easily tell which is the male and which is the female in mammals, but how do we tell which is the male and the female in other organisms?
The individual or organ that produces sperm is always called the male, and the individual or organ that produces the ova is always called the female.
Part 1: Understanding Genes and Inheritance
HSRC Workbook
We use the word ‘egg’ in ordinary life when we refer to structures that have a shell and that contain a developing embryo. A hen egg is an example. In Biology, an egg is the unfertilised female gamete, as well as the structure that contains the developing embryo. The word ovum (plural ova) refers to the unfertilised egg. We will use the term ovum or ova wherever we refer to an unfertilised egg in this unit.
Gametes Male gametes have a head, which contains the nucleus, and, in many species, a long tail. The tail propels the sperm towards an ovum of the same species. Sperm are produced in the testes of animals, or in pollen grains of flowering plants. Look at Figure 21 to see the relative sizes of sperm from some organisms.
Human sperm 1500x
Chicken sperm 500x
Frog sperm 500x
Rat sperm 500x
Figure 21. Sperm from several organisms.
Actual Sizes
Female gametes (ova) have a nucleus, cytoplasm, and variable amounts of stored food. Ova are produced in organs called ovaries in animals and in the ovules of flowering plants. Look at Figure 22 to see the relative sizes of ova of different species.
Human ovum 20x
Frog ovum Human ovum Rat ovum
Coat of jelly Protective membrane Egg membrane Yolky cytoplasm Nucleus
Hen ovum
Figure 22. Ova from several organisms.
Activity 12 1. Work out the actual sizes of the sperm cells in Figure 21. 2. Compare the sizes of ova and sperm of the same species as shown in Figures 21 and 22.
Crossing Over: The Basics of Evolution
39
Meiosis Meiosis (say mayo-sis) is a kind of cell division that results in half the normal number of chromosomes as in a normal cell.
Meiosis is a special kind of cell division that happens before the production of gametes. Remember that mitosis results in two cells with the same number of chromosomes as the parent cell. Meiosis results in four cells with exactly half the number of chromosomes of the parent cells. Every cell in a human body carries 46 chromosomes, but human sperm and ova have 23 chromosomes each. The 46 chromosomes in human body cells consist of 23 pairs, which are called homologous pairs. The two chromosomes of a homologous pair are the same length and shape, and control the same characteristics as each other. Every genetically-controlled characteristic in your body has two genes: one inherited from your mother, and one from your father. The two genes are on the two chromosomes of one homologous pair. Each cell in an organism produced by sexual reproduction carries a double set of chromosomes, one set inherited from the male parent, and the other set from the female parent. Cells that carry a double set of chromosomes are called diploid (2n). During meiosis, the number of chromosomes in the cells is reduced from the diploid number to half of that number. The cells now carry only one chromosome from each homologous pair. We say these cells are haploid (n).
Activity 13 1. Figure 23 shows the process of meiosis. How many chromosomes are present in the parent cell at the start of meiosis? ________ 2. How many chromatids are present at the start of meiosis? ________ 3. How many chromosomes are present in each cell at the end of meiosis? ________ 4. What happens during crossing-over?
5. How many cells are produced at the end of meiosis? ________ 6. What do you notice about the combination of chromosomes in the gametes compared to the chromosomes in the parent cell?
7. Does any gamete carry chromosomes from only one parent? ________
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Part 1: Understanding Genes and Inheritance
HSRC Workbook Diploid (say diployed) means having a double set of chromosomes.
Figure 23. The stages in meiosis.
Haploid (say haployed) means having only one chromosome of each homologous pair.
Crossing Over: The Basics of Evolution
41
Fertilisation
Zygote (say zygoat) is a fertilised ovum.
42
Sexual reproduction involves the act of fertilisation, when the nuclei of the sex cells combine. One sperm enters an ovum of the same species, and the two nuclei fuse. The fertilised ovum is called a zygote. Meiosis and fertilisation are significant events in the production of genetic variation in the offspring of two parents. l During meiosis, the homologous chromosomes separate randomly into the gametes. Each gamete receives a mixture of chromosomes inherited from both parents. l Crossing-over means that many chromosomes contain a mixture of genes from both parents. l Fertilisation brings together chromosomes from two different individuals. Through the processes of meiosis and fertilisation, every individual produced by sexual reproduction has its own combination of genes inherited from both parents. Fertilisation is such an important event that many organisms have special mechanisms to ensure that sperm meet ova. Organisms that live in the sea, such as fish, seaweeds, sponges and coral, release their ova and sperm into the water at the same time. The ova release a substance that is attractive to sperm of the same species. Sperm swim to the ova and fertilise the ova in the water. Organisms that live on land have special mechanisms to ensure that the sperm reach the ova.
Part 1: Understanding Genes and Inheritance
HSRC Workbook
Activity 14 1. Match each term with its correct description. __ g amete
a) A kind of cell division that halves the number of chromosomes __ sperm b) A cell that has two sets of chromosomes. __ ova c) A general name for sex cells. __ diploid d) One half of a chromosome in a cell that is about to divide. __ haploid e) A male sex cell. __ meiosis f) A female sex cell. __ chromosome g) Two chromosomes that are the same shape and size. __ chromatid h) A structure that carries hereditary material in the cell. __ homologous pair i) A cell that has only one chromosome from each homologous pair. 2. Complete the table that compares sperm and ova. Sperm
Ova
Small Do not move much Have a food store Have a tail for moving
3. Each muscle cell in a species of lobster has 250 chromosomes. a. H ow many homologous pairs of chromosomes are present in each cell of the lobster? __ b. H ow many chromatids are present in a cell that is about to divide by mitosis? __ c. How many chromosomes are present in each sperm cell? __ d. How many chromosomes are present in each ovum? __ 4. What would happen if sex cells were produced by mitosis instead of meiosis?
Crossing Over: The Basics of Evolution
43
Summary l
l
l
Reproduction
is necessary for the survival of a species, but not for the survival of an individual. Asexual reproduction is the production of new organisms without the fusion of sex cells. All offspring produced by asexual reproduction are genetically identical. Sexual reproduction involves the production of haploid gametes, which fuse to form a diploid zygote. All offspring produced by sexual reproduction vary in their genetic makeup.
Solutions to activities Activity 1 Photosynthesis; carbon dioxide; diffusion. Oxygen; diffusion. Sugar; sugar; cellular respiration; diffuse; leave; enter.
Activity 3 You would not expect to find chloroplasts in the cells of a root because roots do not photosynthesize.
Activity 4 1. The cells are not circular, so choose diameters where you can see the outer limit of the cytoplasm most clearly. Below is an example using five cells, you may choose different cells, but the method of calculation will be the same. Following the method on page 11 measure the scale bar as 34,5 mm long and convert 34,5 mm to μm: 34,5 x 1000 = 34 500 μm To get the magnification of the micrograph, divide the actual measurement of the scale bar (in μm) by the measurement that it represents: 100 μm.
44
34 500 ÷ 100 = 345 The magnification of the micrograph is x 345 Measure 5 cells and obtain the measurements in mm: 30; 22; 14; 14; 15
Part 1: Understanding Genes and Inheritance
HSRC Workbook
Convert each measurement to μm: 30 000; 22 000; 14 000; 14 000; 15 000
Divide each measurement in μm by the magnification factor (345) to get the actual size of each cell: 86,96 μm; 63,77 μm; 40,58 μm; 40,58 μm; 43,48 μm
Calculate the average size of the five cells as follows: (86,96 + 63,77 + 40,58 + 40,58 + 43,48) ÷ 5 = 55,07 μm
2. Using the same method as for question 1, you will find that the micrograph is shown at a magnification of x 1425. Length mm
Length μm
Actual length μm
Width mm
Width μm
Actual width μm
66
66 000
46,32
20
20 000
14,04
36
36 000
25,26
29
29 000
20,35
61
61 000
42,81
55
55 000
38,60
29
29 000
20,34
18
18 000
12,63
32
32 000
22,46
33
33 000
23,16
Mean
31,44
21,76
3. Using the same method as for questions 1 and 2 you will find a magnification of x 7000. Length mm
Length μm
Actual length μm
Width mm
Width μm
Actual width μm
5
5 000
0,71
5,5
5 500
0,79
6
6 000
0,86
4,5
4 500
0,64
5
5 000
0,71
5,5
5 500
0,79
5,5
5 500
0,79
5
5 000
0,71
4,5
4 500
0,64
5
5 000
0,71
Mean
0,74
0,73
4. The animal cells, with an average diameter of 55,07 μm, are larger than the plant cells, which have an average length of 31,44 μm and an average width of 21,76 μm. The bacterial cells are much smaller than plant or animal cells, since they have an average diameter of 0,74 μm.
Crossing Over: The Basics of Evolution
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5. Cell membrane
Allows substances to enter and leave the cell
Nucleus
Carries the hereditary information of the cell.
Vacuole
Stores water, small molecules and waste products.
Mitochondria
Carries out cellular respiration
Chloroplast
Carries out photosynthesis
Endoplasmic reticulum
Manufactures proteins
Activity 5 1. Chromosomes are found in the nucleus of a cell. They are made of DNA and proteins. Chromosomes carry the information necessary for the development and functioning of the whole individual, including the cell. 2. Questions 2 – 5 encourage the reader to think about the key problems of cell and embryonic development that are solved by the chromosomes.
Activity 6 Chromosome
A structure in the nucleus that carries hereditary information.
Spindle
The threads that stretch across a cell that is about to divide.
Chromatid
One half of a chromosome after replication.
Centromere
The place where two chromatids are attached.
Cloning
The process of producing new individuals without fertilisation.
Activity 7 1. a. A – Prophase – the chromosomes are beginning to become visible in the nucleus. B – Metaphase – the chromosomes are lined up on the equator of the cell. C – Anaphase – the chromatids have separated and been pulled to opposite sides of the cell. D – Telophase – a new cell wall has formed between the two daughter cells, and the nuclear membrane has re-formed, but we can still see chromosomes in the nuclei.
b. cell wall cell wall
nucleus
spindle
spindle
chromosomes
chromosomes chromosomes
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Part 1: Understanding Genes and Inheritance
nuclei
HSRC Workbook
3.
After five cycles of cell divisions, there will be 32 cells in the embryo. Each block in the diagram above represents a cell. 4. a. 20, the same number as in the fertilised egg. b. 40 c. 20 5. Replication is an important event in the cell cycle because it ensures that each daughter cell receives a complete set of the hereditary information needed to make a new individual.
Activity 8 This activity is designed to make the reader think about variation in familiar organisms. It is an open-ended activity.
Activity 9 1. Nonhlanhla and Ntombifuthi are identical twins, therefore they have identical genes. 2. Zanele and Zodwa have inherited genes from the same mother and father, but their genes are different. We know this because the two children are not identical in all physical characteristics, like Nonhlanhla and Ntombifuthi.
Crossing Over: The Basics of Evolution
47
Activity 10 1. a. T he figures along the x-axis of the graph show the height categories of the boys. b. The figures on the y-axis show the numbers of boys. c. 157 and 162. d. Eight boys have genes for being more than 165 cm tall at twelve years of age. e. Food has a strong impact on height. Children who do not receive correct nutrition are shorter than those who eat well. f. Height (cm)
155 – 157
Number of 3 boys
157 – 159
159 – 161
161 – 163
163 – 165
165 – 167
167 – 169
169 – 171
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Part 1: Understanding Genes and Inheritance
HSRC Workbook
4. This survey should come close to the 3:1 ratio in the general population. 5. Sometimes we see the zebras in side view, sometimes front view, and sometimes rear view. We therefore need to choose the stripe pattern that is easiest to identify: the pattern of stripes around the rump is quite distinctive in each of the four zebras shown here. 6. Cell; chromosomes; genes; characteristics (any other physical characteristic could be mentioned here, e.g. hair colour, height) the father; the mother; fertilisation; an embryo child / offspring; parents; identical twins; physical
Activity 11 1. All individuals of a species that reproduces asexually are genetically identical because there is never genetic mixing as when a sperm cell from one individual fertilises an egg cell from another individual. Since all the cells are produced by mitosis, there is never mixing and random segregation of chromosomes into gametes, as occurs during sexual reproduction. 2. All individuals of a species that reproduces sexually are genetically distinct because of mixing of genetic material that occurs during crossing over in meiosis, random segregation of chromosomes into gametes, and fusion of two genetically distinct gametes during fertilisation. 3. Asexual reproduction
Sexual reproduction
One parent
Two parents
No special sex cells required
Two special sex cells – sperm and egg – are required
All offspring are genetically identical to
All offspring are genetically distinct from
the parent and to each other
each other and from the parents
Occurs in plants and animals
Occurs in plants and animals
Activity 12 1. You will need a piece of string to work out the actual sizes of the sperm cells. The human sperm is 85 mm long. Its real size is 85 ÷ 1500 = 56,7 μm. The chicken sperm is 77 mm long. Its real size is 77 ÷ 500 = 154 μm The frog sperm is 52 mm long. Its real size is 52 ÷ 500 = 104 μm The rat sperm is 92 mm long. Its real size is 92 ÷ 500 = 184 μm 2. Notice that the human sperm is about three times smaller than the rat sperm! The rat sperm is the largest of the four species, but its ovum is the smallest. The chicken sperm is the second largest, but its ovum is the largest. The frog sperm and ovum are the third largest of the four species. The human sperm is the smallest, and its ovum the second smallest of the four species. In order of size from smallest to largest: Sperm – human; frog; chicken; rat Ova – rat; human; frog; chicken
Crossing Over: The Basics of Evolution
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Activity 13 1. 6 2. 12 3. 3 4. During crossing over, segments of chromosomes from homologous pairs exchange positions. The result is that each chromosome ends up containing a mixture of genes inherited from its male parent and its female parent. 5. 4 6. Each gamete contains one member of each chromosome, but each chromosome contains a mixture of genetic material from the mother and father of the individual producing the gametes. 7. No, each gamete carries a mixture of chromosomes from both parents.
Activity 14 1.
c) Gamete e) Sperm f) Ova b) Diploid i) Haploid a) Meiosis h) Chromosome d) Chromatid g) Homologous pair
2. Sperm
Ova
Small
Larger than sperm
Very active; swim by undulating the tail
Do not move much
No food stored
Have a food store
Have a tail for moving
No tail
3. a. 125 b. 500 c. 125 d. 125 4. Each time fertilisation occurred, the number of chromosomes in the zygote would double. Meiosis is necessary so that the number of chromosomes remains constant after each fertilisation.
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Part 1: Understanding Genes and Inheritance
Contents The Life Sciences curriculum
52
Overview 1 1. Charles Darwin and the voyage of The Beagle
55
59
Reactions to the publication of the theory
59
2. Darwin’s theory of evolution
60
3. What evidence supports Darwin’s theory?
62
The age of the Earth
62
Changing organisms
63
Intermediate forms
68
70
Descent with modification
72
Comparative anatomy supports evolution
73 75
Searching for genes
77
Discovering the nature of genetic material
77
Nucleic acids
78
DNA replication
80
Why is DNA so important?
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RNA
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How does DNA affect the development and functioning of an organism?
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What is the genetic code?
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Protein synthesis: Translating the genetic code into actions
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How does genetic variation arise?
88
Using genetic information to work out evolutionary histories
91
The Human Genome Project
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Resources
94
Solutions to activities
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2
70
Geographic distribution of species
5. Genetics and evolution
Part
57
Developing and publishing the theory
4. Present day evidence of evolution
HSRC Workbook
Introducing evolution
51
The Life Sciences curriculum From 2006, the subject ‘Life Sciences’ will replace the subject known as ‘Biology’. The new curriculum will mean that previous teachers of Biology will have to adapt their teaching methodology to fit the Revised National Curriculum Statement (RNCS) and to work towards outcomes that learners must achieve by the end of the FET phase of schooling. There are three learning outcomes for Life Sciences: Learning Outcome 1: Scientific Inquiry and Problem-solving Skills The learner is able to confidently explore and investigate phenomena relevant to Life Sciences using inquiry, problem-solving, critical thinking and other skills. Learning Outcome 2: Construction and Application of Life Sciences Knowledge The learner is able to access, interpret, construct and use Life Sciences concepts to explain phenomena relevant to Life Sciences. Learning Outcome 3: Life Sciences, Technology, Environment and Society The learner is able to demonstrate an understanding of the nature of science, the influence of ethics and biases in the Life Sciences, and the interrelationship of science, technology, indigenous knowledge, the environment and society. As a Life Sciences educator, you should construct your learning programme so that learners make progress in all three learning outcomes throughout the FET band. The learning outcomes are further described by a set of assessment standards that specify the levels of achievement within each learning outcome in each Grade. The assessment standards are shown in Table 1.
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Part 2: Introducing evolution
Table 1: Learning Outcomes and Assessment Standards for Grades 10 – 12
Assessment Standard
Grade 10
LO1: Scientific Inquiry and Problem-solving Skills
1. Identifying and Identify and question questioning phenomena phenomena. and planning an investigation
HSRC Workbook
Learning Outcome
Grade 11
Grade 12
Identify phenomena involving one variable to be tested.
Generate and question hypotheses based on identified phenomena for situations involving more than one variable.
Plan an investigation using instructions
Design simple tests to measure the effects of this variable.
Design tests and/or surveys to investigate these variables.
Consider implications of investigative procedures in a safe environment.
Identify advantages and limitations of experimental design.
Evaluate the experimental design.
2. Conducting an investigation by collecting and manipulating data
Systematically and accurately collect data using selected instruments and/ or techniques and following instructions.
Systematically and accurately collect data using selected instruments and/or techniques.
Compare instruments and techniques to improve the accuracy and reliability of data collection.
3. Analysing, synthesising, evaluating data and communicating findings
Display and summarise the data collected. Analyse, synthesise, evaluate data and communicate findings.
Select a type of display that communicates the data effectively.
Manipulate data in the investigation to reveal patterns. Identify irregular observations and measurements. Allow for irregular observations and measurements when displaying data.
Compare data and construct meaning to explain findings.
Critically analyse, reflect on and evaluate the findings. Explain patterns in the data in terms of knowledge.
LO2: Construction and Application of Life Sciences Knowledge
Draw conclusions and recognise inconsistencies in the data.
Provide conclusions that show awareness of uncertainty in the data.
Assess the value of the experimental process and communicate findings.
Suggest specific changes that would improve the techniques used.
1. Accessing knowledge
Use a prescribed method to access information.
Use various methods and Use various methods and sources to sources to access information. access relevant information from a variety of contexts.
2. Interpreting and making meaning of knowledge in Life Sciences
Identify concepts, principles, laws, theories and models of Life Sciences in the context of everyday life.
Identify, describe and explain concepts, principles, laws, theories and models by illustrating relationships. Evaluate concepts, principles, laws, theories and models.
LO3: Life Sciences, Technology, Environment and Society
Interpret, organise, analyse, compare and evaluate concepts, principles, laws, theories and models and their application in a variety of contexts.
3. Showing an understanding of the application of Life Sciences knowledge in everyday life
Organise, analyse and interpret concepts, principles, laws, theories and models of Life Sciences in the context of everyday life.
Analyse and evaluate the costs and benefits of applied Life Sciences knowledge.
1. Exploring and evaluating scientific ideas of past and present cultures
Identify and investigate scientific ideas and indigenous knowledge of past and present cultures.
Compare scientific ideas and Critically evaluate scientific ideas indigenous knowledge of past and indigenous knowledge of past and present cultures. and present cultures.
2. Comparing and evaluating the uses and development of resources and products, and their impact on the environment and society
Describe different ways in which the uses and development of resources and products, and their impact on the environment and society.
Comparing and evaluating the uses and development of resources and products, and their impact on the environment and society.
3. Comparing the influence of different beliefs, attitudes and values on scientific knowledge
Analyse and describe Compare scientific ideas and the influence of different indigenous knowledge of past beliefs, attitudes and values and present cultures. on scientific knowledge and its application to society.
Crossing Over: The Basics of Evolution
Evaluate and present an application of Life Sciences knowledge.
Comparing and evaluating the uses and development of resources and products, and their impact on the environment and society.
Critically evaluate and take a justifiable position on beliefs, attitudes and values that influence developed scientific and technological knowledge and their application in society.
53
In an outcomes-based framework, the content is the vehicle that educators use to facilitate learners’ achievement in each assessment standard. The curriculum statement provides a list of content topics that must be covered, but it provides very little detail about the depth and breadth of each topic. This means that educators must have good subject matter knowledge in Biology, as well as access to a wide range of resources so that they are able to construct learning experiences that are interesting and valid in Life Sciences. The content topics in the RNCS for Life Sciences are arranged in four knowledge areas: l Tissues, cells and molecular studies l Structures and control of processes in basic life systems l Environmental Studies l Diversity, change and continuity. The core knowledge comprises 80% of the content to be covered. The remaining 20% must be used by the teacher to incorporate local knowledge into the curriculum, or to adapt specific knowledge to local circumstances. Part 2 of this workbook will help you to build your own understanding of topics that fall within two of the knowledge areas: Diversity, change and continuity, and Tissues, cells and molecular studies. The knowledge area ‘Diversity, change and continuity’ requires South African teachers and learners to include a basic understanding of evolution in the curriculum for Life Sciences. This is a new and very welcome addition to school Biology in South Africa.
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Part 2: Introducing evolution
HSRC Workbook
Overview Learning Outcome 3: Life Sciences, Technology, Society and the Environment (Grade 12) Assessment Standard: We know this when the learner is able to critically evaluate scientific ideas and indigenous knowledge of past and present cultures. Knowledge area: Diversity, Change and Continuity Content topics: Changes of knowledge through contested nature and diverse perceptions of evolution. Beliefs about creation and evolution.
For thousands of years, people have asked questions such as: l How did the world begin? l Where do plants and animals come from? l Where do I come from? l Where do humans come from? l What is the purpose of life? l Is there life in any other part of the universe? If you belong to a particular faith, your answers will probably reflect your faith. Every faith or culture in the world has a belief about the beginning of the world, the origin of humans and the purpose of life. Some faiths believe in a god or gods who created the world and everything in it. Other faiths or cultures believe the world developed from nothing, without a god or gods who created the world. Before entering a discussion about evolution, it is important to understand the difference between belief by faith, and proof by following the scientific process. l When we believe something by faith, we accept that it is true without requiring evidence to support the belief. l The scientific process requires that we seek explanations for natural phenomena by examining the evidence, by experimenting, and then explaining our findings in the light of what we have discovered and what we already know. In other words, science is constantly looking for explanations for what we see in our world. The question that people often ask is whether this means that scientists cannot also hold religious beliefs. And the answer is, of course they can! Through science we learn many wonderful things about the world. As a religious scientist, you may marvel at the wonders of your god or gods. A faith or set of cultural beliefs provide guidelines for daily living and interacting with other people. Many scientists derive great personal comfort from their religious beliefs. Occasionally, though, scientific findings conflict with people’s religion or traditional beliefs. In your work as a Science or Biology teacher you may have found certain concepts or topics that conflict with your own or the learners’ beliefs. In our work at the University of KwaZulu-Natal, we have experienced many such concepts. For example, many South Africans believe that albino people do not die, but simply disappear. Science tells us this is not possible, yet we have found this belief to be very widespread. Other examples include that AIDS is caused by witchcraft or that it is a punishment sent by God, that humans are reincarnated as animals, so we should not harm any other living creatures and that we should not eat foods from certain animals either because they are considered sacred or unclean.
Crossing Over: The Basics of Evolution
55
Homo sapiens is the genus and species (taxonomic classification) of modern humans. Sapiens is the only existing species of the genus Homo.
56
As teachers we need to respond to confl icts between science and religious or traditional beliefs. The most effective way to do so is to openly acknowledge the beliefs, and to offer the scientifi c explanation as an alternative explanation. The subject of Part 2 of this workbook is evolution. The theory of evolution proposes that all the living organisms on Earth today have developed slowly and gradually from other organisms. The theory is supported by a huge amount of evidence from scientifi c studies. Most modern biologists believe that evolution has occurred and continues to infl uence life on Earth, but biologists differ on some of the details of evolution. Scientists don’t yet know everything there is to know about the process of evolution, but so far, they have found no evidence that contradicts the theory. The theory of evolution is an example of a scientifi c theory that has confl icted with major religions and with cultural beliefs about a single creation event. The accumulated evidence from many different sources now supports the idea that there was not a single creation event, and that evolution is a much more likely explanation. The major religions of the world have now accepted that the huge amount of scientifi c evidence in favour of evolution is convincing, and that the Earth and life on it have been and still are undergoing gradual changes. Modern geologists have calculated that the Earth is at least 4 500 million years old. Biologists have gathered evidence that life on Earth has developed from simple organisms such as bacteria, which fi rst lived on Earth about 3 800 million years ago, to the present variety of organisms that share our Earth at present. Modern humans (Homo sapiens) are fairly recent in terms of Earth’s history: there is no evidence of that Homo sapiens existed until about 100 000 years ago. Most species of organisms that once lived on Earth are now extinct, which means that there are no longer any living individuals of that species on Earth. In this part of the workbook you will be able to gain only a brief insight into a fraction of the evidence on which the theory of evolution is based. If you want to fi nd out more, you can read much more around the subject since this is the most important theory of modern Biology. There are also some excellent videos that cover the history of life and these make interesting viewing. South Africa has played, and continues to play, an extremely important role in the development of evolutionary theory. We have many outstanding fossils in different parts of the country, including some of the oldest known fossils, and some excellent fossils that tell us about human origins. Young South Africans deserve to know about this research and to be proud of our country’s contribution to this fi eld of scientifi c research. As the new curriculum comes into effect over the next few years, it will be possible for Life Science teachers to introduce learners to the evidence and underlying concepts of evolution. Evolution is the most important unifying theory in modern Biology. It enables us to make sense of most of our observations about life, and the patterns we see in living organisms and life processes.
Part 2: Introducing evolution
HSRC Workbook
1 Charles Darwin and the voyage of The Beagle Learning Outcome 3: Life Sciences, Technology, Society and the Environment (Grade 12) Assessment Standard: We know this when the learner is able to critically evaluate scientific ideas and indigenous knowledge of past and present cultures. Knowledge area: Diversity, Change and Continuity Content topics: History and nature of science. Changes of knowledge through contested nature and diverse perceptions of evolution.
The theory of evolution is usually associated with the name of Charles Darwin. Charles Darwin was born in England in 1809. His father and grandfather were both doctors, so young Charles was sent to study medicine at Edinburgh University. However, Charles was not interested in medicine, so he changed his course and began to study theology so that he could become a minister. He was always very interested in nature and collected shells, birds’ eggs, beetles and plants. Darwin never completed his training for the ministry, because in 1831 he was invited to join an expedition which was going on a round-the-world voyage. The purpose of the voyage was to investigate life on other continents, and to map parts of the world that had not yet been mapped. Charles Darwin was only 22 years old, and he was very excited to be invited to join the team as its naturalist. The ship was called The Beagle, and during its fi ve-year voyage, Darwin saw many strange organisms that infl uenced his ideas about the origin of the Earth, and the origins of life on Earth. You can see the route taken by The Beagle during its fi ve-year voyage in Figure 2.
Figure 1. Charles Darwin.
Figure 2. The route taken by Charles Darwin and the crew of the The Beagle.
Crossing Over: The Basics of Evolution
57
Darwin and the crew left England in 1832 and travelled first to Brazil in South America. They then made their way down the east coast of South America, visiting islands along the coast and stopping at various ports on the mainland. They rounded the southernmost tip of South America and sailed up the west coast. As you can see from the map, they made many stops as they mapped the coastline of South America. As they travelled, Darwin collected rocks, plants and animals from all the places they visited. The last stop before they left South America was a group of islands called the Galapagos Islands. Darwin’s visit to these islands was very important for his later ideas about evolution. After leaving South America, The Beagle crossed the Pacific Ocean, stopping at various islands. They stopped at the northernmost point of New Zealand, made several stops in Australia, and then sailed across the Indian Ocean to visit Mauritius, Madagascar and South Africa. From the Cape of Good Hope, The Beagle crossed the South Atlantic Ocean back to Brazil before returning to England. Darwin had collected hundreds of specimens from each country and island they visited. Each specimen was carefully labelled with details of exactly where and when he collected it. He noticed many curious phenomena, but it was another twenty years before he published his book on evolution by natural selection. Charles Darwin was not the first scientist to think that living organisms present today have evolved from previous living things. His grandfather, Erasmus Darwin, had published articles in which he presented his ideas that all presentday species have evolved by a gradual process of change from pre-existing species. However, before Charles Darwin, no-one could explain the mechanism of evolution. During his travels, Charles Darwin marvelled at the way living things are adapted to their surroundings. ‘Adaptation’ means ‘well-suited’ or ‘adjusted’. For example, we say that desert plants are adapted to living in dry areas. They have adaptations such as spines instead of leaves or thick fleshy leaves or welldeveloped root systems. Darwin was strongly influenced by a geologist called Charles Lyell. Lyell published a book called ‘Principles of Geology’ in 1830, in which he proved that the Earth is much older than the age calculated by theologians. Lyell proposed that the Earth was hundreds of millions of years old, not a few thousand, as most people thought. Charles Lyell is now considered to be the father of modern geology. Darwin took a copy of Charles Lyell’s book with him on the voyage of The Beagle, and he was able to confirm that geological features of the Earth form by slow acting forces that are still at work. Most of the Earth’s rocks form from fine particles of sand and silt that sink to the bottom of lakes, rivers and seas. The sediments change into rock as pressure from the water and more layers of sediment increases. Sedimentary rocks are still forming. Sedimentary rocks usually contain layers or strata with the oldest layers at the bottom and the most recent strata at the top. It takes thousands of years to form a layer, so a sedimentary rock hundreds of metres thick must be millions of years old. Darwin also collected fossils during his voyage around the world. Fossils are the remains of organisms preserved in sedimentary rock. Darwin was amazed to find fossils of marine organisms in mountains more than 4 000 metres above the present sea level. Darwin realised that the mountains must have once been below sea level. Observations such as these confirmed Lyell’s proposal that the surface of the Earth is slowly but constantly changing, and is shaped by natural forces such as Earthquakes, volcanoes, wind, and water. 58
Part 2: Introducing evolution
HSRC Workbook
Developing and publishing the theory After returning from the voyage of The Beagle, Darwin spent many years studying all the specimens he had collected on his journey. He was convinced that evolution does occur, and he began to form and refine his idea about how it happens. He called the mechanism of evolution ‘natural selection’. Because his theory conflicted with the Old Testament creation story and he wanted to present as much supporting evidence as he could, he delayed making his ideas known for as long as possible. However, in 1858 he received a scientific paper written by a young British naturalist, Alfred Wallace. Wallace had been working in southeast Asia, and he had arrived at the same conclusion as Darwin: living organisms evolve by a process of natural selection. The arrival of Wallace’s paper forced Darwin to complete his book called ‘On the Origin of Species by Means of Natural Selection’ in 1859.
Reactions to the publication of the theory Darwin and Wallace announced their theory of evolution by natural selection together, since both had independently arrived at the same hypothesis, supported by evidence found in two different parts of the world. As expected, the book caused an outcry from the press, the Church and the general public, but it was sold out within hours of arriving in the bookshops. Although many people were able to accept that other organisms evolve, they found it hard to believe that humans have also evolved. Darwin wrote a book called ‘The Descent of Man’ in 1871. The book extends the theory of evolution to include human evolution. Although no fossil humans or pre-humans had then been found, Darwin suggested that the most likely place for such fossils was Africa. As it turns out, Darwin was right! The first fossil to shed light on the evolution of humans was found in South Africa in 1929. Since then, many fossils have been found in South Africa and East Africa, and many gaps in the story of human evolution have been filled. Darwin was not a healthy person for most of his life. He died in 1882, and was buried in Westminster Abbey. Since Darwin’s death, the theory of evolution by natural selection has been refined and changed as new evidence becomes available. In the next section we will study what Darwin proposed about the nature of evolutionary change. Remember that at the time that he wrote ‘The Origin of Species’, chromosomes and genes had not yet been discovered.
Activity 1 Arrange these sentences describing Charles Darwin’s life in the correct sequence: 1. Darwin published a book called ‘The Descent of Man’. 2. He studied medicine at Edinburgh University. 3. Charles Darwin was born in 1809. 4. He visited the Galapagos Islands, where he saw many new species of plants and animals. 5. Alfred Wallace published a paper describing his theory of evolution by natural selection. 6. Charles Darwin published a book called ‘On the Origin of Species by Means of Natural Selection.’ 7. He joined an expedition to map parts of the world. 8. Charles Darwin died and was buried in Westminster Abbey.
Crossing Over: The Basics of Evolution
59
2 Darwin’s theory of evolution Learning Outcome 2: Construction and application of Life Sciences knowledge. Grade 12 Assessment Standards: We know this when the learner is able to use various methods and sources to access relevant information from a variety of contexts. We know this when the learner is able to interpret, organise, analyse, compare and evaluate concepts, principles, laws, theories and models and their application in a variety of contexts. Knowledge area: Diversity, Change and Continuity Content topics: Evolution theories, natural selection.
The word extinct refers to species that no longer live on the Earth.
In his book ‘On the Origin of Species by Means of Natural Selection’, Darwin explained that he believed that all organisms living on Earth today have slowly and gradually evolved from other organisms that are now extinct. We know that many extinct organisms once existed on Earth because scientists have found their fossils.
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Figure 3. Darwin’s theory of natural selection.
60
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Part 2: Introducing evolution
Activity 2 Try to name a breed or variety that has the following characteristics: 1. A maize plant that produces white seeds. __ 2. A type of cattle bred especially for meat. __ 3. A kind of chicken bred for egg-laying. __ 4. A variety of bean plant that grows into a small bush. __ 5. A variety of orange that has no seeds. __
Crossing Over: The Basics of Evolution
61
HSRC Workbook
The way in which evolution occurred was called ‘natural selection’. Darwin’s theory of natural selection was based on several key ideas, which are illustrated and explained in Figure 3. l Many more organisms are produced than survive to adulthood. For example, a tree produces thousand of seeds, but only a few will germinate. A fish lays thousands of eggs, but only a few will hatch into young fish. l Individuals of a population compete for resources. For example, the seedlings of a tree in a forest compete with other plants for space, sunlight, water and nutrients. Young fish will compete with each other for space, food and shelter. Some individuals will survive, but many will die. l Individuals produced by sexual reproduction vary within a species. For example, there are millions of people in South Africa, but we all look different (except for identical twins). Cattle in one herd differ in the colours of their coats. Some differences are not visible: some maize plants in a field may be more resistant to disease than others. l Those individuals within a population that are best adapted to their environment will survive to reproduce. Individuals that are less well adapted will die before they reach adulthood, or will not reproduce at all. This process is sometimes called ‘the survival of the fittest’. l Through differential reproduction rates, the characteristics that are best adapted to the environment will gradually spread through the population. Thus, although Darwin knew nothing about genes and heredity, he was aware that characteristics were passed on from one generation to another. l The process of natural selection results in a slow and gradual change in a population as it becomes progressively better adapted to its environment. If A population is a group of organisms of the same species living in one area. the environment remains unchanged for a long time, the population should also remain unchanged. However, a change in the environment may cause some species to become extinct, and other species to change. l Darwin believed that if natural selection continued for long enough, it would result in a new species. He believed it took thousands or millions of years for one species to change into a different species. Darwin used the example of artificial selection to illustrate the process of natural selection. If a plant breeder wants to produce plants with particular characteristics, for example maize plants that produce extra large cobs, she collects seeds from plants that have the biggest cobs. She grows the seeds from these plants, and again chooses the plants that have the biggest cobs. By doing this repeatedly, she can produce plants that have much bigger cobs than the plants she started with. The process of artificial selection has resulted in an improvement in all the food crops we grow commercially. It is also responsible for changes in domestic animals, such as cows that are specially bred to produce lots of milk, chickens that grow very fast and can be slaughtered at the age of six to eight weeks, and sheep that produce thick layers of wool.
What evidence supports Darwin’s theory? Learning Outcome 2: Construction and application of Life Sciences knowledge (Grade 12) Assessment Standards: We know this when the learner is able to use various methods and sources to access relevant information from a variety of contexts. We know this when the learner is able to interpret, organise, analyse, compare and evaluate concepts, principles, laws, theories and models and their application in a variety of contexts. Knowledge area: Diversity, Change and Continuity Content topics: Biological evidence of the evolution of populations. Fundamental aspects of fossil studies.
Darwin could show that selective breeding brought about changes in a population, but he couldn’t prove that artifi cial selection could produce new species. Although many different breeds of cattle now exist, all the breeds can still interbreed and produce fertile young. However, if you allow a horse and a donkey to interbreed, they produce a sterile offspring called a mule. This proves that horses and donkeys are separate species, but different breeds of cattle are all one species. Darwin believed that natural selection has to work for a very long time before a new species forms. The human life span of about 70 years is too short to witness the evolution of new species.
The age of the Earth So, one of the fi rst things that Darwin needed to show to convince people that his theory was true, was that the Earth could be millions of years old. This had to be the case if there was to be enough time to produce the huge variety of life forms that we see today. In Darwin’s time, people believed the Earth was 4 000 years to 20 000 years old. According to Darwin’s theory, this would not have been enough time for evolution to have occurred. However, after Charles Lyell published his book, in which he proposed that the Earth was really hundreds of millions of years old, Darwin had the support he needed for his theory of evolution. The Earth constantly changes its form due to very slow movements of its surface. Occasionally we become aware of these movements through earthquakes and volcanoes. Modern techniques of dating rocks have shown that the Earth is about 4 500 million years old. Samples of rock collected on the moon have confi rmed the age of the Earth and the moon as 4 500 million years. The moon formed at the same time as the Earth, so its rocks should be the same age as the Earth’s rocks.
62
Part 2: Introducing evolution
HSRC Workbook
Changing organisms The second thing Darwin needed to support his theory of evolution was to show that organisms change over time. This requirement was met by studying fossils. Most organisms start to decay once they die. After some time, there is no trace of their existence. However, sometimes an organism dies in a place where it does not decay immediately, but is preserved and becomes fossilised. Sometimes the dead organism is preserved by freezing, or by becoming embedded in a hard substance that oozes from trees. However, most fossils occur in sedimentary rock. Figure 4 shows how fossils form in sedimentary rock.
Fault in rock Younger fossil in young rock
Dead organism
Organisms die. Their bodies sink to the sea bed and become buried in the sediment.
Skeleton fossilised
Erosion of rock exposes fossils.
Dead organism Older fossil in deeper rock
As the sediment hardens into rock, the remains turn to stone. New layers form over the old.
The rocks fold and are raised up out of the sea. They are now exposed to wind and rain
Erosion and faults in the rock expose the different layers and the fossils they contain.
Figure 4. The process by which fossils are formed.
A dead organism sinks to the bottom of a sea or lake. The body is covered with layers of sediments such as sand, mud or silt. Further layers are added to the first layer, building up enormous pressure on the dead body. Slowly, the sediment turns to rock, slowing down the decomposition of the dead body. As the body slowly decomposes, the decayed parts are replaced by minerals. The dead organism turns into stone in the newly-formed sedimentary rock. Once this happens we say a fossil has formed. Soft tissues like internal organs do not fossilise well, but bones, teeth, external skeletons and woody tissues of plants do fossilise well. We even find fossilised dung, fossilised nests, and fossilised footprints of animals! After many thousands or millions of years under the water, the sedimentary rocks may be exposed due to the Earth’s crust moving. Weathering by wind, rain and temperature changes may result in the fossils being exposed and discovered by humans. Many fossils have been discovered during road-building or in quarries. As the sedimentary rocks are blasted away, fossils that have lain undisturbed for millions of years may be exposed.
Figure 5. A fossilised ammonite from northern KwaZulu-Natal.
Crossing Over: The Basics of Evolution
63
Using modern techniques for dating the rocks, scientists can work out the ages of fossils found in the rocks. Dating techniques have become very accurate, and scientists always cross-check using two or more techniques before they publish the age of a particular fossil. Fossils had been known for hundreds of years before Darwin began his work. People believed that fossils were the remains of organisms that still existed ‘somewhere’ in the world. Some of the fossils of enormous animals like dinosaurs and woolly mammoths were so big that they could not easily hide away from humans. However, as travellers explored more and more parts of the Earth, it became clear that these organisms no longer existed – they were in fact extinct, and the only evidence that they ever existed was in the form of their fossilised remains.
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Figure 6. Number of taxonomic groups found at different times in Earth’s history.
According to Darwin’s theory, the simplest organisms should be found in the oldest rocks. One should be able to see a gradual increase in complexity of the organisms, and also an increase in diversity of life as we examine fossils from successively younger rock layers. After carefully examining millions of fossils, scientists have found that, in general, Darwin was correct. Figure 6 shows the numbers of different families at various times in the history of the Earth. (When we talk about ‘families’ in this way, we mean groups of species, not human families.)
64
Part 2: Introducing evolution
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HSRC Workbook
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Figure 7. Increases and decreases in species over time.
Figure 7 shows all groups of life linked to the first cells on Earth (at the bottom of the diagram). The width of the strand shows the number of species in that group at a particular time. When a strand gets wider, it means that the number of species in that group has increased. We say the group has radiated. When a strand narrows, it shows that the number of species decreases. Many species become extinct at that time. Sometimes a whole group becomes extinct.
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Table 1: A history of life
Era
Period
Epoch
MYA
Major biological and geological events
Cenozoic
Quaternary
Pleistocene
1.8
First modern humans
Tertiary
Pliocene
7
Miocene
26
Origin of the first human-like forms
Oligocene
38
Monkey-like primates appear
Eocene
54
Paleocene
65
Small mammals undergo adaptive radiation
145
Major extinction of the dinosaurs and much marine life
Mesozoic
Cretaceous
Flowering plants appear Jurassic
210
Large dinosaurs dominate the Earth Pangaea (super continent) forms First birds
Triassic
250
Small dinosaurs appear First mammals Insects become more diverse
Paleozoic
Permian
285
Major extinction occurs. Most species disappear Conifers appear
Carboniferous
360
First reptiles and arthropods Horsetails, ferns and seed-bearing plants abundant Major insect diversity arose
Devonian
410
Age of the fishes Fish with bones and jaws appear Amphibians appear Major extinction of marine invertebrates and fishes Insects first appear
Silurian
430
Notochord becomes flexible as single rod is replaced with separate pieces as seen in the ostracoderms (armoured fish without bones, jaws or teeth) Plants invade the land
Ordovician
505
First vertebrates appear Major extinction of marine species First fungi appear
Cambrian
520
Major extinction of the trilobites Origin of the main invertebrate phyla
Proterozoic
2600
Multicellular eukaryotic animals appear First eukaryotic cells appear Oxygen producing bacteria present, atmosphere and oceans oxygenated
Archean
4600
Stromalites formed Chemical evolution resulting in the formation of first cells First rocks formed Earth is born
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Activity 3 Figures 6 and 7 and Table 1 are three different ways of representing the history of life. Use all three sources to find the answers to the following questions: 1. How long ago did the first cells appear on Earth? __________ 2. Figure 6 shows that the number of taxonomic families dropped suddenly at the end of the Permian period. (a) Look at Table 1 to see what happened at that time. (b) Find a group of animals that started to radiate 550 million years ago, and became extinct 360 million years ago.
3. W hich group of plants was dominant (had the widest strand) between 435 and 240 million years ago?
4. W hich group of woody plants was dominant between 240 and 65 million years ago?
5. W hich group of woody plants is dominant at present? When did this group of plants begin to diversify?
6. M any species of molluscs became extinct during a certain time period. What was that time period?
7. D uring which geological periods did the following classes of Arthropoda begin to radiate: a. Crustaceans
b. Chilopoda and diplopoda
c. Insects
d. Arachnids
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8.
Which group of animals has the greatest number of species at present?
9.
Work out the sequence in which vertebrate classes evolved. VERTEBRATE GROUP
MILLIONS OF YEARS AGO
1 2 3 4 5
10. R eptiles show two large-scale extinctions. Estimate the geological ages at which these extinctions occurred. ________ and ________ 11. W hich vertebrate group radiated between about 350 million and 290 million years ago, and then almost became extinct? _____________ 12. Which vertebrate groups evolved most recently? ________________ 13. W hat can you say about the diversity of animals compared to the diversity of plants on Earth today?
14. W hat can you say about the number of taxonomic families on Earth today as compared with 500 million years ago?
15. We seldom find fossils of fungi, protista and annelids. Can you say why?
Intermediate forms Darwin’s evidence from fossils was criticised because there seemed to be so many gaps as life evolved from one group to another. For example, birds show many similarities to fossil dinosaurs, so it was thought that birds probably evolved from some kind of dinosaur. However, at the time that Darwin wrote his book, no such intermediate fossil had been found. In 1860-1861, a fossilised animal was found which had many dinosaur features, such as teeth and a long tail, but it had feathers and it could probably glide.
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Figure 8 shows a fossil and a reconstruction of the dinosaur-bird that was named Archaeopteryx. Several more fossils of this half-dinosaur, half-bird have now been found. Archaeopteryx is thought to be a link between dinosaurs and birds.
Figure 8. Fossil Archaeopteryx and reconstruction.
Intermediate fossils are rare. Darwin explained this by saying that of all the organisms present on Earth at one time, only a very few left fossils. Many fossils will never be discovered by humans, because they lie buried deep in sedimentary rocks. Darwin expected that the gaps would be filled in as more fossils are discovered. He said that trying to work out the evolution of a particular group from the fossil record is like trying to read a book with most of the pages missing. Since Darwin’s death, many intermediate fossils have been discovered, and scientists have been able to reconstruct the evolution of several modern species, such as horses, giraffes, elephants and camels. One of the most intensivelyresearched fossil sequences relates to human evolution. South Africa has played an important role in discovering fossils that help us to understand our origins.
Activity 4 Carry out some of your own research using the library or the Internet to find out about: a. The Cradle of Humankind b. Mrs Ples c. Little Foot. You will find the website address www.cradleofhumankind.co.za very useful.
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4 Present day evidence of evolution Learning Outcome 1: Scientific inquiry and problem-solving skills (Grade 12) Assessment Standard: We know this when the learner is able to: systematically and accurately collect data using selected instruments and/or techniques; manipulate data to reveal patterns; explain patterns in the data in terms of knowledge Knowledge area: Diversity, Change and Continuity Content topics: Planning, conducting and investigating plants and animals – a comparison. Learning Outcome 3: Life Sciences, Technology, Society and the Environment (Grade 12) Assessment Standard: We know this when the learner is able to critically evaluate scientific ideas and indigenous knowledge of past and present cultures. Knowledge area: Diversity, Change and Continuity Content topics: Adaptation and survival
In the previous section, we used evidence from the geological history of the Earth to show that evolution has occurred. In this section, we will study some evidence from organisms alive on Earth today to support evolutionary theory.
Geographic distribution of species Without an understanding of evolution, we could believe that each species was created to suit its environment. From a human perspective, species seem to be fi xed; that is, they have always been just as we see them now. This is partly because evolution takes much, much longer than any of our life spans. If it takes several thousand years for a new species to evolve, it is extremely unlikely that we will see a new species evolve in one lifespan of 70 years. As Darwin travelled round the world, he was struck by the enormous diversity of plants and animals throughout the world. The mammals of Australia are completely different from those of Europe and Africa, which are different from those of South America or North America. The large island of Madagascar has its own unique set of life, unlike those found in any other part of the world. The sheer diversity of life astounded Darwin. We now know that there are at least three million species on the Earth, and some scientists say there could be as many as 30 million.
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Figure 9. Large fl ightless birds are found on different continents.
If we include all the species that are now extinct, there could have been as many as 3 000 million different species of life since Earth was formed! Darwin noticed that each continent had animals and plants with similar modes of life, but in each continent the species were different. For example, large fl ightless birds are found in Africa (ostrich), Australia (emu), South America (rhea) and New Zealand (moa). Darwin wanted to fi nd an explanation for this – he asked: Why, if there was a single creation event, are there so many different species with the same mode of life? There are many examples of this kind of duplication.
Figure 10. A kangaroo with a joey in its pouch.
Did you know? The duplication of species with the same mode of life does not occur within continents, but Darwin noticed that many organisms within a continent share some characteristics. For example, all the mammals occurring naturally in Australia are marsupials. We do not fi nd marsupials in Africa, North America, Europe or Asia.
Crossing Over: The Basics of Evolution
A marsupial is a mammal, found principally in Australia, Central and South America. The females of most species do not have a placenta, instead they have a marsupium – a large abdominal pouch, which contains the mammary glands and shelters the young.
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Adaptive radiation means that a number of new species evolve from the original species (the ancestor) that enters a new environment. Each new species becomes adapted to a different mode of life in that environment. All the new species are descended from the original species, and are modified from the original species. Hence, descent with modification.
The large ground finch has a large, strong, crushing beak ... like large nutcrackers.
Descent with modification Darwin explained the patterns of distribution of plants and animals in different parts of the world in terms of two concepts: descent with modification, and adaptive radiation. His observations of island biogeography helped him to develop these concepts. We have previously mentioned Darwin’s visit to the Galapagos Islands, about 1 000 kilometres off the west coast of South America. These islands presented Darwin with excellent evidence of how descent with modification and adaptive radiation may have worked. The islands are a group of volcanic islands that arose from the sea between one and five million years ago. When they first emerged from the ocean, they would have been lifeless lumps of rock. When Darwin visited the Galapagos Islands, he found plants and animals there that occur nowhere else on Earth. The most spectacular animals are the tortoises, which can grow to about 150 kg in mass. Each island has its own species of tortoise, which is slightly different from the others. Among the bird species on the Galapagos Islands, Darwin was particularly interested in the finches. There are thirteen different species of Galapagos finch, each adapted for eating different kinds of food and having different modes of life. Some finch species occur on only one or two islands. Figure 11 shows some of the finch species, and the ways in which they are adapted for eating different kinds of food.
The large tree finch has a strong, sharp beak for grabbing and cutting ... like metal cutters.
The warbler finch has a small pointed beak, for probing into cracks ... like tweezers.
The small ground finch, has a small but strong, crushing beak ... like small nutcrackers.
The cactus finch, has a long, tough beak, for probing ... like longnosed pliers.
Figure 11. Galapagos finches are adapted to different modes of life.
Darwin noticed that finches also occur on the mainland of South America, 1 000 km away from the islands. Was it possible that the Galapagos finches were descended from the mainland finches? The finches are too small to have flown 1 000 km across the sea, but at times very strong winds blow from the mainland out across the sea. The winds sometimes carry birds away from their normal habitat and far out into the ocean. A few mainland finches could have landed on the islands during such a storm. The original lost birds would be the ancestors of all present thirteen species. 72
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HSRC Workbook
Darwin used this reasoning to explain all the patterns of similarity and difference that we observe in the two million species of life that live on Earth at present. Each group of species, sharing a number of similarities, is in fact descended from a common ancestor. We can detect these patterns of similarity, and therefore shared ancestry, in all groups of living organisms. We can extend this line of reasoning right back to the origin of life, because all living organisms share some characteristics, like these ones: l All living organisms are made of cells. l All cells contain genetic material in the form of DNA. l All living systems use energy to maintain their internal organisation. We can use patterns of similarity and differences to work out the evolutionary relationships among groups of species. Working out the evolutionary relationships forms a large part of the research work of biologists. We try to make the connections between organisms, using evidence from the structure and functioning of the organisms. The more similarities we find, the more closely related the species are.
Comparative anatomy supports evolution Many biologists before Darwin had noticed the similarities in the anatomy (structure) of species that belong to the same phylum. Among animal species, there are about 30 different body plans, each one representing a different phylum. Shared characteristics were the basis of the science of taxonomy. Darwin used comparative anatomy to support his theory of evolution. For example, all vertebrates share the same basic structure in their limbs. You can see the basic pattern of the vertebrate limb in Figure 12. BASIC PATTERN OF THE PENTADACTYL LIMB
HUMAN ARM
BAT (used for flying)
One upper limb bone Humerus BIRD (used for flying) Two lower limb bones Radius Ankle or wrist bones Digits, fingers or toes
Ulna Carpals Metacarpals Digits
MOLE (used for digging)
Figure 12. Basic anatomy of the vertebrate limb.
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Activity 5 1. Look at the forelimb of a bat in Figure 12. a. Write the names of the bones on each label line. b. How does a bat’s forelimb differ from that of a human?
c. A bat moves by fl ying. In what ways is a bat’s forelimb adapted for fl ying?
2. Look at the forelimb of a mole. a. Write the names of the bones on each label line. b. A mole digs burrows in the soil with its forepaws. In what ways is the forelimb adapted for digging?
3. L ook at the forelimb of a bird. Identify each bone in the bird’s wing. In what ways is it different from the human forelimb?
b. Why do you think the bird’s forelimb has this structure?
4. B ats and birds are both fl ying vertebrates, yet their wings have different structures. What differences do you see in the structure of the wings of bats and birds? What does this tell you about the ancestors of bats and birds?
The similarities in the basic structure of vertebrate limbs strongly suggest that all vertebrate species have evolved from a single ancestral species. The descendants of this ancestral species evolved in different ways. Each species is adapted to its way of life. 74
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HSRC Workbook
5 Genetics and evolution Learning Outcome 2: Construction and application of Life Sciences knowledge (Grade 12) Assessment Standard 1: We know this when the learner is able to use various methods and sources to access relevant information from a variety of contexts. Assessment Standard 2: We know this when the learner is able to interpret, organise, analyse, compare and evaluate concepts, principles, laws, theories and models and their application in a variety of contexts. Knowledge area: Tissues, cells and molecular studies. Content topics: Genes, inheritance; Chromosomes; DNA, protein synthesis; Meiosis, production of sex cells. Learning Outcome 3: Life Sciences, Technology, Environment and Society (Grade 12) Assessment Standard: We know this when the learner is able to analyse and evaluate different ways in which resources are used in the development of biotechnological products. Knowledge area: Tissues, cells and molecular studies. Content topics: Research in a field of biotechnology.
Darwin realised that for evolution by natural selection to work, there had to be some way of transferring characteristics from one generation to the next. He had no idea that these characteristics were controlled by genes, and that genes pass from one generation to the next. Gregor Mendel (1822–1884) lived at about the same time as Darwin, but the two men never met. Mendel investigated the transfer of characteristics from one generation to the next by breeding different kinds of plants. He noticed, for example, that some pea plants produced only green peas, while other plants produced only yellow peas. He conducted a series of experiments with pea plants by cross-pollinating plants with yellow seeds with plants with green seeds (called the parent generation.) He collected the seeds from these cross-pollinated plants, and grew them into adult plants. The adult plants only produced yellow seeds. It seemed as if the green seed characteristic had completely disappeared in this generation of plants (called the fi rst fi lial or F1 generation.) Mendel allowed the plants of the F1 generation to self-pollinate. He collected the seeds, planted them and allowed them to grow to maturity. These plants were called the second fi lial or F2 generation. When the F2 plants produced seeds, he found that one quarter of the seeds were green, while three-quarters were yellow. Mendel counted thousands of seeds before he came to these proportions. It seemed as if the green colour had not completely disappeared in the F1 generation, because it reappeared in the F2 generation. Mendel’s work showed that characteristics are indeed passed on from one generation to another, and they do not necessarily blend in each generation.
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Alleles in plants
Variety 1 Self-fertilising
Variety 2 Self-fertilising
Alleles in seeds
PARENT GENERATION
All yellow seeds
All green seeds Variety 1 pollinated by Variety 2
Variety 2 pollinated by Variety 1
Cross pollination Alleles in pollen
Allele in egg
F1 HYBRID GENERATION Allele in egg
Hybrid plant selffertilises (any hybrid seed from either plant will give the same result)
Alleles in egg Alleles in pollen Alleles in seeds
Alleles in plant
Alleles in seeds
All yellow seeds
3 yellow seeds F2 HYBRID GENERATION 1 green seed
Figure 13. Mendel’s experiment with peas.
Mendel’s work was not recognised until long after his death. In the early 1900s, researchers repeated his experiments with plants and with mice. They confirmed Mendel’s findings and proved that physical characteristics do indeed pass from one generation to the next. Researchers gave the name ‘genes’ to the factors that control inheritance. Sometimes a characteristic is an ‘either/or’ characteristic, like the colours of the peas. They are either yellow or green: they are never anything in between. We say that there are two forms of the gene that controls seed colour in peas, and we call the forms alleles. The allele that produces yellow seeds is dominant over the allele that produces green seeds, which is a recessive allele. Some alleles cause blending of characteristics inherited from both parents. For example if a red bull mates with a number of white cows, all the calves will have a mixture of red and white hairs. If a red-and-white bull mates with a number of red-and-white cows, then one quarter of the calves will be red, one quarter will be white and the remaining half of the calves will be red-and-white.
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Evolutionary biologists realised that the nature of hereditary material (or genes) was very important in understanding how characteristics are inherited. They began to search for the genes, which held the hereditary information. For some years, scientists had seen tiny threads within every cell which they called chromosomes. They noticed that every time a cell divided by mitosis, the chromosomes doubled in number, and then divided equally between two cells. Thus, it seemed that the chromosomes could be the carriers of the genes from one generation of cells to the next. They imagined that the genes were like beads strung out along the chromosome threads. Variations could result from slight differences in the genes in different individuals. By the 1920s, scientists had worked out that evolution was mainly to do with different alleles. Physical variation within a species was due to differences in alleles; therefore if natural selection favoured a particular phenotype, the frequency of those alleles would increase. Evolution could be explained in terms of changes in the frequencies of alleles that produced the favourable characteristics. The process of combining new ideas from genetics with the older Darwinian theory or evolution was called the new synthesis. Ronald Fisher, a British mathematician, was responsible for much of the early work on the new synthesis. He showed that slight changes in the genes could result in slow and gradual changes to physical appearance (phenotypes) over time. Most of his work was done mathematically, since at this stage no-one knew the nature of the genetic material. Most physical characteristics are controlled by many different genes, with each single gene having a small effect on the overall phenotype. Fisher claimed that the small effects produced by changes in a single gene resulted in small alterations in the phenotype. Natural selection could then act through certain phenotypes being more successful than others. Over the next ten to twenty years, the new synthesis was refined as more experiments were carried out. Fruit-flies played an important part in the research. Fruit-flies have only four chromosomes, and they have giant chromosomes in their salivary glands. The giant chromosomes can be more easily studied than normal chromosomes. Scientists have been able to map all the genes of the giant chromosomes of fruit flies. By changing certain genes, they can show that the genes control particular characteristics of the phenotype.
The genotype is the particular combination of genes that an organism carries on its chromosomes. The phenotype is the expression of the genes, which we see as the physical appearance of the organism.
Discovering the nature of genetic material The next major step in the development of the new synthesis was the discovery of the chemical structure of chromosomes. In 1944, scientists discovered that chromosomes contain DNA. DNA has a relatively simple structure, consisting of nucleotides made up of a molecule of sugar, a phosphate group, and a base. There are only four kinds of bases: adenine, thymine, guanine and cytosine. The bases can follow each other in any order in a strand of DNA. Suddenly, scientists realised that the arrangement of bases in DNA could provide a code which formed the basis of the code of life. Changing the sequence of bases would change the genetic code. However, scientists still didn’t know how the DNA was transferred from one cell to another, nor how it could affect the phenotype.
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HSRC Workbook
Searching for genes
The structure of DNA was finally revealed by James Watson and Francis Crick in 1953. DNA is a double helix, like two strands of rope twisted together. This discovery was massively important to biologists and we will spend some time describing the structure of DNA before moving on.
Nucleic acids Nucleic acids, such as DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are built up from units called nucleotides. Figure 14 shows the structure of a nucleotide. e]dhe]ViZ \gdje
E
8='
dg\Vc^X WVhZ
WVhZ
/ D
*"XVgWdc hj\Vg
Figure 14. General structure of a nucleotide.
The central part of a nucleotide is a 5-carbon sugar. The sugar in RNA is ribose, while in deoxyribonucleic acid, the sugar is deoxyribose.
Activity 6 Look at the molecules of ribose and deoxyribose. Count the number of atoms of carbon, hydrogen and oxygen in each molecule and fill in the table below. :QJW[M
Molecule
Number of carbon atoms
,MW`aZQJW[M
Number of hydrogen atoms
Ribose Deoxyribose
What is the difference between the two molecules?
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Number of oxygen atoms
HSRC Workbook
A phosphate group is attached to the sugar molecule. The phosphate group has the structure:
OH
O
P
P
OH
Which is abbreviated as:
The phosphate group plays an important role in linking nucleotides. The third part of the nucleotide molecule is an organic base, which is linked to the sugar ring. There are five different organic bases: adenine (A); guanine (G); cytosine (C); thymine (T); and uracil (U). Four of the bases, adenine, thymine, guanine and cytosine occur in DNA. RNA contains adenine, guanine, cytosine and uracil. Cells continually produce nucleotides, which form a ‘pool’ that can be used to manufacture DNA, RNA or other substances in the cell.
direction of sugar-phosphate backbone
direction of sugar-phosphate backbone
purines – A = adenine pyrimidines – T = thymine
G = guanine C = cytosine
Figure 15. Base-pairing in nucleic acids.
Molecules of nucleic acids are built up from long chains of nucleotides linked together, as you can see in Figure 15. The bonds between the sugar and phosphate molecules are strong, but the bonds linking the organic bases are weak hydrogen bonds.
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Notice that the bases A, T, G, and C are each drawn a particular shape. Adenine can only form hydrogen bonds with thymine, and guanine can only bond with cytosine. In RNA, the pairs are adenine-uracil and guanine-cytosine. It is impossible for A to pair with G, or C to pair with T. To summarise: nucleic acid molecules consist of two strands of nucleotides linked to each other by sugar-phosphate links and base-pairing. The two strands twist together to form a stable, three-dimensional structure called a double helix.
DNA replication New copies of DNA can be made in a simple way. Study Figure 16 carefully to see how a double strand of DNA can make a matching copy of itself. complementary strands of DNA
DNA replicating enzyme
building units
template new chain
Figure 16. DNA replication in a double helix.
complementary strands
The two strands of the double helix of DNA separate and new nucleotides fit in to match the exposed bases. The enzyme DNA polymerase catalyses the reaction. The new nucleotides bond to each other by sugar-phosphate links and bond with the old DNA strand by base-pairing. Thus the DNA strands make exact copies (replications) of themselves.
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DNA replication is essential because DNA carries the genetic instructions that must be passed on to the daughter cells. During a normal cell cycle, the DNA replicates so that when the chromosomes separate during mitosis, each daughter cell gets a complete set of genetic instructions.
Activity 7 The diagram below shows a portion of a DNA molecule that is beginning to replicate. Draw in the nucleotides that will be attached to each separated strand of DNA.
Why is DNA so important? l
l
l
DNA
is an extremely stable molecule that carries the coded genetic information for the cell. The genetic information remains undamaged as it passes from one cell generation to the next. DNA can replicate so it can make new copies of itself for each new cell. The genetic information is passed on to every new cell. DNA controls the cell’s activities through the synthesis of proteins. Various types of RNA help DNA to make proteins.
RNA RNA molecules are always single-stranded, and are usually much shorter than DNA molecules. Cells contain three important types of RNA.
1. Messenger RNA Messenger RNA, also called mRNA, is produced in the nucleus from instructions encoded in the DNA. mRNA passes into the cytoplasm where ribosomes attach themselves to it. The process of protein synthesis begins with mRNA. mRNA molecules have 75 – 3 000 nucleotides and are not folded.
2. Transfer RNA Transfer RNA, or tRNA are small RNA molecules containing 75 – 90 nucleotides. The molecules are folded to give the molecule a specific shape. Figure 17 shows the shape of a tRNA molecule as it would look if it was ‘flattened out’.
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One loop has a special place (the anticodon) where it attaches itself to an mRNA molecule. At the opposite side of the molecule is a site where tRNA combines with an amino acid. There are about twenty different amino acids in cells, and 64 different kinds of tRNA molecules. Each kind of tRNA can attach to one particular amino acid. general clover-leaf structure (molecule ‘flattened out’)
single stranded 75 – 90 nucleotides folded to give precise shape, act as intermediates between mRNA and protein
amino acid
base-paired regions
‘anticodon’ loop (combines with mRNA molecule)
extra loop not present in all tRNAs
Figure 17. Transfer RNA.
The function of tRNA is to ‘translate’ the instructions from DNA into a sequence of amino acids forming a protein chain.
3. Ribosomal RNA Ribosomal RNA, or rRNA is made in the nucleolus of a cell’s nucleus. rRNA is a major component of the small particles called ribosomes. rRNA molecules are very large molecules consisting of thousands of nucleotides. Each molecule is a single strand of RNA.
How does DNA affect the development and functioning of an organism? This question was answered by two French scientists, Francois Jacob and Jacques Monod in 1961. They discovered that RNA acts as a ‘messenger’, taking copies of sections of DNA to places in the cell where proteins are made. The code is translated into a sequence of amino acids forming the primary structure of a protein molecule. Thus the genetic code influences the phenotype of an organism by instructing the cell to make certain proteins at certain times. The proteins then form new structures in the cell, or cause reactions to take place, or are secreted to function elsewhere in the organism.
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What is the genetic code?
HSRC Workbook
During the 1960s a huge amount of research was carried out on the genetic code. Eventually, the secret of the genetic code was discovered in the sequences of the bases in DNA.
Did you know? Francois Jacob, Jacques Monod and Andre Lwoff were jointly awarded a Nobel Prize in 1965 for their contributions to genetics and work on messenger RNA.
A gene on a DNA molecule is like a recipe that contains instructions for a protein molecule. The gene itself is simply a sequence of nucleotides on one strand of a DNA molecule. The genes also act like ‘switches’. Each gene has a promoter region that signals the ‘start position’. At the other end of the gene is a stop signal marking the end of the gene. In the human genome, most genes may be divided up into sections, with non-coding sections in-between the coding sections.
long molecule of DNA comprising one chromosome DNA double helix gene region (untwisted for clarity)
one gene
adjacent regions of double helix
start signal
hydrogen bonds to complementary bases on non-coding strand
coding strand carries instructions stop for protein synthesis signal sequences of bases on coding strand
complementary noncoding strand carries no instructions complete base-pairing between strands triplet code words – each word specifies the position of an amino acid in the corresponding protein chain
start signal
Figure 18. Information coding in DNA.
There are four bases in DNA: adenine (A), thymine (T), guanine (G) and cytosine (C). The bases make the codes used for protein synthesis. A sequence of three nucleotides forms a ‘word’, also called a triplet word or codon. Each triplet word codes for one specific amino acid. Therefore, the genetic code is simply the sequence of bases in the strand of DNA!
Activity 8 Using the letters A, T, G and C, see how many different three-letter codons you can form. Remember that you can use any combination of three letters.
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Did you know? Scientists are now paying attention to finding out more about ‘junk’ DNA. They argue that it cannot be ‘junk’ otherwise the evolutionary process would have got rid of it. They have also found that it is virtually identical across species.
The amazing thing about DNA is that the same code applies to ALL living organisms! The differences between organisms can be traced back to differences in the sequences of the bases in the genes. Your own genome consists of about 3,2 billion bases, which you have inherited from your parents. The bases are organised into about 31 000 genes, which carry all the information that has directed the development and functioning of your body from the time you were a single-celled zygote to the present time. Less than 2% of your genome actually codes for proteins: the rest is called ‘junk’ DNA. Because each one of us is unique, we each have slight differences in our DNA (unless you have an identical twin). Variation, which was so important in the theory of natural selection, comes about because of these slight variations in the coding of the DNA.
Protein synthesis: Translating the genetic code into actions The development and functioning of cells is controlled by protein molecules, which are either structural proteins or enzymes. Proteins consist of chains of amino acids, of which there are twenty in cells. The amino acids can join in any sequence to form a protein, and each kind of protein contains a specific sequence of amino acids. By controlling protein manufacture, DNA therefore controls all the activities of the cell. But how does a sequence of bases in DNA inform the structure of a protein molecule?
Transcription DNA controls protein synthesis in two important stages, called transcription and translation. Study Figure 19 which is a simple summary of protein synthesis. nucleus nuclear envelope
nuclear pore nucleolus
DNA messenger transfer RNA RNA transcription coded instructions copied from DNA to RNA
Figure 19. Summary of protein synthesis.
translation interpretation of instructions and assembly of molecules
405
ribosome subunits
605
messenger RNA amino acids
Part 2: Introducing evolution
ribosome growing protein chain
ATP cytoplasm
84
ribosomal RNA
HSRC Workbook
DNA is normally held inside the nucleus. When a gene becomes active, its genetic code is transcribed (changed into a different form) into mRNA, which leaves the nucleus and enters the cytoplasm. An enzyme called DNA-dependent RNA polymerase catalyses the process of transcribing the genetic code. The enzyme recognises the ‘start’ position on the DNA molecule and attaches itself to the DNA at this point. This seems to break the hydrogen bonds that hold the two DNA strands together. The two DNA strands move apart in the region of the gene. The enzyme moves along the DNA coding strand and joins RNA nucleotides in a sequence that matches the DNA bases. Figure 20 shows how transcription occurs.
complementary non-coding strand start signal
double helix unwound by enzyme
growing mRNA molecule formed on DNA template complementary mRNA molecule being formed
direction of transcription
stop signal coding strand of DNA
enzyme molecule DNA-dependent RNA polymerase
metabolic ‘pool’ of available nucleotides appropriate nucleotides added from ‘pool’ base-pairing in enzyme region
corresponding mRNA codons coding strand of DNA triplet code words RNA polymerase enzyme
Figure 20. Transcription.
When the enzyme finds the base T on the DNA strand, an A is added to the RNA molecule. If the next DNA base is G, C is added to the RNA. RNA does not contain thymine (T), so if the DNA base is A, uracil (U) is added to the RNA molecule. The final mRNA is exactly complementary to the DNA plan or template. At the end of transcription, the enzyme recognises the ‘stop’ code and separates from the DNA. The DNA molecule rewinds to its normal double helix shape. mRNA moves out of the nucleus through a nuclear pore into the cytoplasm.
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Translation The information coded by mRNA is translated into a protein molecule. tRNA and ribosomes must be present for translation. Remember that there are at least twenty different kinds of tRNA and 20 different amino acids in the cytoplasm of a cell. Each tRNA molecule links with its own amino acid and takes part in protein synthesis. Ribosomes are small cell organelles that contain rRNA and protein. It is difficult to describe the process of translation in a simple way, so you should study the four stages (A, B, C and D) in Figure 21 carefully as you read this section. l
l
l
l
Table 2: The genetic code
A
– The ribosome locks on to one end of the mRNA molecule and begins translation. Each ribosome has two sites, labelled ‘tRNA sites’ in the diagram, where tRNA can attach to mRNA. B – The tRNA molecule with the complementary code to the first mRNA codon attaches to mRNA at is binding site. Remember that each tRNA has an amino acid attached at the opposite side of the molecule. C – The second tRNA molecule comes into position next to the first tRNA molecule and both form base pairs with the mRNA molecule. The amino acids attached to the tRNA molecules join each other, and are linked together by an enzyme. D – The ribosome moves along the mRNA molecule and lines up tRNA molecules to match the mRNA codes. The amino acids attached to the tRNA molecules join the growing protein chain. When an amino acid has joined the protein chain, its tRNA molecule leaves the mRNA and picks up another amino acid.
It takes about one minute for a ribosome to travel the length of an mRNA molecule. Usually several ribosomes move along the mRNA molecule at the same time so that several protein molecules are made simultaneously. The ribosome drops off when it reaches the end of the mRNA molecule and releases its protein molecule. Remember that there are 64 possible triplet words, and only twenty amino acids. Scientists have worked out what the triplet words are for all the amino acids. They are listed in Table 2.
Amino acid
mRNA codons used to specify Amino acid the position of this amino acid
mRNA codons used to specify the position of this amino acid
Alanine
GCA, GCC, GCG, GCU
Lysine
AAA, AAG
Arginine
AGA, AGG, CGA, CGC, CGG, CGU
Methionine
AUG (start codon)
Asparagine
AAC, AAU
Phenylalanine
UUC, UUU
Aspartic acid
GAC, GAU
Proline
CCA, CCC, CCG, CCU
Cysteine
UGC, UGU
Serine
AGC, AGU, UCA, UCC, UCG, UCU
Glutamic acid
GAA, GAG
Threonine
ACA, ACC, ACG, ACU
Glutamine
CAA, CAG
Tryptophan
UGG
Glycine
CGA, GGC, GGG, GGU
Tyrosine
UAC, UAU
Histidine
CAC, CAU
Valine
GUA, GUC, GUG, GUU
Isoleucine
AUA, AUC, AUU
‘stop’ codons
UAA, UAG, UGA
Leucine
CUA, CUC, CUG, CUU, UUA, UUG
TOTAL NUMBER OF DIFFERENT mRNA
64
CODONS
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ribosome 60S subunit
A
tRNA sites initiation codon
mRNA codons
ribosome binding site ribosome 40S subunit
mRNA molecules
first tRNA in position
amino acid
B
tRNA molecule tRNA anticodon
initiation codon
mRNA molecule amino acid
C First amino acid linked to second by internal ribosome enzyme
tRNA molecule tRNA anticodon
ribosome moves along mRNA strand
D
Growing polypeptide chain-amino acids linked by peptide bonds free end of mRNA molecule
mRNA codons
ribosomes moves along mRNA strand
Figure 21. Translation.
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Activity 9 1.
How many triplet words specify the following amino acids? a. lysine ________ b. arginine ________ c. tryptophan ________ d. isoleucine ________
2. A n mRNA molecule is shown below. Write the name of the correct amino acid below each triplet word.
start AUG\ACA\AUC\ACC\UAU\UUG\ACA\CUU\CUA\
stop 3. The genetic code for a gene as represented in the DNA is: ATG\TGT\TAT\AAC\TAG\ACT\GGC\
a. Transcribe the gene into an mRNA molecule.
b. Translate the mRNA molecule into a protein molecule.
Once you know the fundamental mechanism of heredity, you can understand the importance of genes in evolution. l Genes control all the physical characteristics and the functioning of an organism. l Variations in the genes produce variable phenotypes. l Natural selection then operates on the phenotypes. Those individuals that are best adapted to the environment are more likely to survive and reproduce. Their genes pass to their offspring, while the genes of individuals that are not so well adapted will not pass on to offspring because they will not reproduce.
How does genetic variation arise? Variation among individuals is a key element in the theory of evolution by natural selection. Genetic variation causes phenotypic variation. You inherited your genes from your parents, who in turn inherited their genes from your four grandparents, who inherited their genes from your eight great-grandparents, and so on. Three events in our life cycle result in genetic variation amongst individuals. To understand all three we need to revise the events occurring during meiosis. Figure 22 shows the process of meiosis.
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Figure 22. The process of meiosis.
Stage 1. Chromosomes divide into pairs of identical chromatids joined to one another by a centromere. Centrioles move to opposite ends of the cell. Protein fibres form around each of them, just as in mitosis.
Stage 2. Chromosomes pair up. The two chromosomes of a pair are called homologous chromosomes. Notice the exchange of a segment of one chromosome with the corresponding segment of its homologous chromosome.
Stage 4. Homologous pairs of chromosomes separate. Each chromosome (consisting of two chromatids) of a pair moves to opposite ends of the cell.
Stage 5. The chromosomes gather into two bunches. The cell begins to divide and a new nuclear membrane forms around each bunch.
Stage 7..The nuclear membranes disappear and new spindles form at right angles to the first. The chromosomes (still as pairs of chromatids) arrange themselves on the equator.
Stage 6. The cell divides.
Stage 8. The centromeres divide. The chromatids separate and bunch at opposite ends of each cell. The chromatids are now the new chromosomes. Each cell begins to divide.
Stage 3. The nuclear membrane disappears. A spindle forms between the centrioles just as in mitosis. Homologous pairs of chromosomes arrange themselves at the equator.
Stage 9. A nuclear membrane forms around each bunch of chromosomes and the cells divide.
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Event 1 – Independent assortment of chromosomes at meiosis Remember that each cell in your body contains two sets of chromosomes, one set inherited from your mother, and one set inherited from your father. During meiosis, the matching chromosomes from the mother and father come together in homologous pairs. At the end of the first meiotic division, the pairs separate, and move to opposite poles of the cell. The direction in which a chromosome moves is independent – it does not matter whether it came from the mother or the father! The result is that the chromosomes at each pole are a mixture of chromosomes inherited from the mother and the father. Every egg cell and every sperm cell contains a mixture of chromosomes inherited from the mother and father of the person producing the eggs or sperm.
Event 2 – Crossing over at meiosis At the beginning of meiosis, homologous pairs line up side by side. Pieces of chromosomes break off and move from one chromosome to its homologous pair. Crossing over can occur at several places along a homologous pair. The result is that each chromosome can contain a mixture of genes from the mother and the father. Crossing over is a major source of variation in the genotypes of plants and animals.
Event 3 – Fertilisation between gametes from two different individuals We have already shown that genetic variation arises during meiosis, so that cells produced by meiosis do not contain identical genetic material. Meiosis often results in the production of gametes, that is, eggs and sperm. Thus the eggs or sperm that you produce contain a combination of genes that you inherited from your parents. At fertilisation, the genetic material from two different individuals combines to form a zygote. The new individual will have a unique combination of genes, different from each of its parents, and different from any other offspring produced by the same parents.
Mutation The methods above only rearrange alleles of existing genetic material. New alleles can only be produced by a mutation. A mutation is any change in the sequence of bases in the DNA, which is transmitted to the next generation. Mutations occur quite frequently, but most have no effect on the phenotype. Mutations can be caused by a change in a single gene, or a whole chromosome. Many mutations are lethal, that is, the organism containing the mutation dies. However, occasionally a mutation occurs, which has a beneficial effect on the phenotype. Beneficial mutations play a very important part in generating evolutionary change.
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In the previous unit, we showed that each individual has a unique combination of alleles that gives him or her a unique phenotype. However, we know that individuals of a species can only produce young of their own species. Human parents can only produce human babies, cows can only produce calves, pawpaw plants can only produce more pawpaw plants, and so on. Thus each species must have a combination of genes that belongs only to its own species. When taxonomists group species into genera, they usually use the external appearance to make decisions about which species belong to one genus, and which species belong to a second genus. For example, horses, donkeys and zebras share many similarities, so they are placed in one genus, Equus. Cattle share some similarities with horses, donkeys and zebras, but they also have some distinct differences. Cattle are therefore placed in a separate genus, Bos. All living organisms have DNA in their cells. DNA has the same basic structure, whether it comes from an elephant or a bacterium. All the characteristics of an organism (its phenotype) are encoded in its DNA (its genotype), and this information is transmitted in the chromosomes from one generation to the next. We know that the phenotypes of organisms are controlled by their genotypes. Therefore, we should expect that the genotypes of horses, donkeys and zebras should be different from each other, but not as different as the genotype of cattle is from horse, donkeys and zebras. Over the past ten to twenty years, scientists have developed ways of extracting DNA and comparing it with DNA from another species. The technique is called DNA hybridisation. The first step is to extract DNA from the cells of each species. The DNA is ‘cut’ into short pieces using enzymes. Then the double strands of DNA are forced to unwind and separate into single strands. The single stranded DNA from two different species is mixed together. Each strand will try to find a matching strand. The more similar the DNA of the two different species is, the more likely their DNA is to make mixed (hybrid) double strands, with one strand coming from each species. The hybrid strands will separate when they are heated. The more similar two strands are, the higher the temperature needed to separate the strands. By measuring the temperature at which strands separate, we can work out how similar the DNA of two species is. For example, suppose hybrid strands of DNA from horses, donkeys and zebras separate at these temperatures: Horse-donkey: 50˚C Horse-zebra: 55˚C Donkey-zebra: 56˚C
Taxonomy is the theory, principles and process of classifying organisms into established categories using their observed similarities and evolutionary relationships.
These results tell us that horses are more closely related to donkeys than to zebras. We can show the relationships in a branching diagram:
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HSRC Workbook
Using genetic information to work out evolutionary histories
Mammoths looked like hairy elephants. They lived in North America and Asia.
Did you know? There is currently a project underway to re-breed the quagga at various sites in South Africa. You can find more information about this project from Iziko Museums. www.iziko.org.za
A similar technique has been used to work out the relationships among different human races. The results tell us that all humans are genetically very similar: in fact, there is much more similarity among all human populations than in our most closely-related species, chimpanzees. Comparing the DNA of different human populations tells us that although all human populations are so similar genetically, Africans have more genetic diversity than other races. This supports the hypothesis that Homo sapiens evolved in Africa, and only migrated to other parts of the world much later. Thus we can use differences and similarities in DNA to work out possible evolutionary relationships of present-day organisms. An exciting new development is that we can extract DNA from extinct organisms that have been preserved intact by freezing or stored in museums. Some mammoths were frozen in thick layers of ice in northern Asia thousands of years ago. Samples of DNA have been taken from these frozen, partly decayed creatures and compare it with the DNA of modern elephants. Mammoths were closely related to modern African and Asian elephants. The quagga is an African mammal that became extinct early this century. The quagga looked like a zebra, but it had fewer stripes. Scientists were able to obtain samples of DNA from the skin of a stuffed quagga in the South African Museum. They proved that the quagga had the same DNA as other zebras, and therefore it was probably not a different species.
The Human Genome Project The Human Genome Project set out to map the sequence of bases in all the genes of humans. The project has been an international project, with workers at laboratories in many countries of the world. Volunteers donated blood samples or semen samples, and scientists extracted the DNA from the samples. The project was first talked about in the mid-1980s but was only started in 1990. At first, it took a year or more to work out the sequence of 12 000 bases. With a total number of three million bases, it was initially estimated that the project would take fifteen years. Since then, the process has been streamlined and automated, so it actually took only fifteen months to produce a ‘working draft’ of the whole human genome. The first draft was published in February 2001, and will be followed by a more accurate high-quality draft. The database generated by the human genome project is available to the general public through the following websites: http://www.ensembl.org/genome/central and http://www.ncbi.nim.nih.gov/genome/central The aim of the Human Genome Project was to develop genomic tools and resources, with the eventual goal of identifying genes that cause human disease and disorders. If we are able to identify the genes that cause disease and other disorders, we may be able to develop techniques for correcting defective genes so that medicine as we know it will be revolutionised. Such a powerful source of knowledge brings with it enormous ethical and legal implications. For biologists, though, the tools that have been developed through the human genome project help us to work out in much greater detail the evolutionary history of the great diversity of life.
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Activity 10 Find out about new fields in biological research: a. What is bioinformatics?
b. What is biotechnology?
c. What is genetic engineering?
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Resources If you want to find out more about the topics covered in this workbook, the resources listed below will be useful to help you. However, this field of Biology is developing so rapidly that you should make sure that any book you buy has been updated in the last five years or the information is likely to be out of date. l
Burnie, D. 1999. Get a Grip on Evolution. Weidenfeld & Nicolson, London.
l
A
l
Dennis,
l
Contains
l
Mackie,
concise and well-written introduction to evolutionary theory, with a review of supporting evidence. C. & Gallagher, R. (Eds). 2001. The Human Genome. Nature Palgrave, Basingstoke, England.
articles for the lay person as well as the original scientific papers describing key aspects of the human genome project. R. 2000 Ape-Man: The Story of Human Evolution. BBC Worldwide Ltd,
London.
94
l
Beautifully
illustrated coverage of the unfolding story of human origins, including some of the unsolved mysteries of human evolution. The video series by the same name has some interesting footage.
l
MacRae,
l
Contains
l
All
C. 1999. Life Etched in Stone: Fossils of South Africa. The Geological Society of South Africa, Johannesburg. photographs of many of the fossils that have been found in South Africa. A wonderful resource book for finding out about fossils in your home area. modern Biology textbooks have a section describing the concepts of evolution and reviewing the supporting evidence. Get the most recent that you can.
Part 2: Introducing evolution
HSRC Workbook
Solutions to activities Activity 1 3. 2. 7. 4.
Charles Darwin was born in 1809. He studied medicine at Edinburgh University. He joined an expedition to map parts of the world. He visited the Galapagos Islands, where he saw many new species of plants and animals. 5. and 6. Alfred Wallace published a paper describing his theory of evolution by natural selection. Charles Darwin published a book called ‘On the Origin of Species by Means of Natural Selection’. 1. Darwin published a book called ‘The Descent of Man’. 8. Charles Darwin died and was buried in Westminster Abbey.
Activity 2 There are several correct answers for each of these questions.
Activity 3 This is quite a difficult activity that will only be suitable for use with advanced learners. 1. 2. 3. 4. 5. 6.
About 3 000 million years ago (answer from Figure 7) a. A major extinction occurred at the end of the Permian period. b. Trilobites. Bryophytes Gymnosperms Angiosperms: they began to radiate about 90 – 100 million years ago. Mollusc diversity decreased between 360 and 290 million years ago, which corresponds to the Carboniferous Period. 7. a. Crustaceans began to radiate between 505 and 435 million years ago, which corresponds to the Ordovician Period. b. Chilopoda and Diplopoda radiation began between 410 and 360 million years ago, which corresponds to the Devonian Period. c. Insect radiation began in the Devonian Period. d. Arachnid radiation also began in the Devonian Period. 8. The insects have the greatest number of species at present.
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9. Vertebrate group
Millions of years ago
1.
Fish
505
2.
Amphibians
380
3.
Reptiles
360
4.
Mammals
138
5.
Birds
138
10. Two large-scale extinctions of reptiles have occurred at 240 – 220 million years ago (during the Triassic) and at 55 – 65 million years ago (end of the Cretaceous / beginning of the Tertiary Period). 11. The Amphibians radiated during the Carboniferous Period and then almost became extinct. 12. Birds and Mammals. 13. Animal diversity, in terms of number of different major groups, is much greater than plant diversity. This may not be true for numbers of species within each group. 14. There are almost the same number of major groups as there were 500 million years ago. The trilobites became extinct, but all other major groups were present 500 million years ago and are still present. 15. These are soft-bodied organisms which rarely form fossils, therefore they are relatively rare in the fossil record.
Activity 4 If you visit the website, choose ‘Educator’ and then read ‘The Cradle of Humankind’ and ‘Discoveries and Debates’ fact sheets.
Activity 5 1. A bat’s forelimb differs from a human’s forelimb in that the bones are much thinner, and the digits (fingers) much longer than a human’s fingers. There is a web of skin that connects all the fingers together to make a wing. The web of skin joining the digits of a bat make it possible for the bat to fly. 2. T he bones are short, thick and strong, with projections for the attachment of the strong muscles necessary for digging in soil. The fingers end in long claws that help with digging. There is an extra claw on one of the metacarpals. 3. T he bird’s forelimb has only three digits and fewer metacarpals, two of which are elongated and fused. The bones are long and slender. The bird’s forelimb gives rigidity and lightness, through the reduction in the number of bones and the fusion of some metacarpals.
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4. T he contrasting anatomy of bird’s and bat’s wings is a good example of convergent evolution, where two separate groups have evolved parallel structures for a particular function – in this case, flying. Notice that in bats, the ability to fly is associated with skin connecting very elongated digits, while in the bird the same function is associated with reduced digits, and feathers that provide the surface area required for flight.
Activity 6 Ribose has five carbon atoms, ten hydrogen atoms, and five oxygen atoms. Deoxyribose has five carbon atoms, ten hydrogen atoms, and four oxygen atoms. Differences: Deoxyribose has one less oxygen atom than ribose. The hydrogen that is lost is from carbon atom 2 in the ring structure, where an hydroxyl ion (OH) is replaced by a hydrogen atom (-H).
c
g
a
t a
c
a
g
c
t
t
a
t g
c a
a
t
g
t
Activity 7
Activity 8 Second letter
First letter
A
T
G
C
Third letter
A
AAA
ATA
AGA
ACA
A
T
TAA
TTA
TGA
TCA
G
GAA
GTA
GGA
GCA
C
CAA
CTA
CGA
CCA
A
AAT
ATT
AGT
ACT
T
TAT
TTT
TGT
TCT
G
GAT
GTT
GGT
GCT
C
CAT
CTT
CGT
CCT
A
AAG
ATG
AGG
ACG
T
TAG
TTG
TTG
TCG
G
GAG
GTG
GTG
GCG
C
CAG
CTG
CTG
CCG
A
AAC
ATC
AGC
ACC
T
TAC
TTC
TGC
TCC
G
GAC
GTC
GGC
GCC
C
CAC
CTC
CGC
CCC
T
G
C
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Activity 9 1. 2.
a. lysine: two triplet codes b. arginine: six triplet codes c. tryptophan: one triplet code d. isoleucine: three triplet codes methionine – threonine – isoleucine – threonine – tyrosine – leucine – threonine – leucine – leucine – stop 3. a. UAC\ACA\AUA\UUG\AUC\UGA\CCG b. tyrosine – threonine – isoleucine – leucine – isoleucine – stop (notice that this would be a portion of a protein molecule, because it does not begin with the ‘start’ codon).
Activity 10 The topics listed here are growing rapidly as technology improves. Search the Internet or recent library books for up-to-date information on each topic.
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