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
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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
xiii
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.
vi
Introduction
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
vii
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.
viii
The Natural Sciences curriculum
HSRC Workbook
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
ix
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
xi
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.
lWithin our country’s borders, we have at least seven hotspots of biodiversity,
where we can see the results of rapid evolution in particular areas.
lWe 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.
lWe 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.
lThrough 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.
xii
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
xiii
xiv
HSRC Workbook
Understanding
Genes&Inheritance
Part
1
Contents
Overview
3
1.Thecell
4
Thestructureofacell
5
2.Measuringverysmallstructures 9
Usingscalebars
10
14
Cellsdivide Eventsinthecellcycle
14
15
4.Thechromosomesandcelldevelopment 16
5.Mitosis
Stepsintheprocess Celldivision 3.Thecellcycle
19
19
21
23
23
24
25
26
28
28
32
32
34
34
34
6.Inheritance
Parentsandoffspring
Whatisaspecies?
Chromosomescarrygenes Thegeneticbasisofinheritance
Therelationshipbetweenchromosomes
andthewholeorganism
Measuringvariation 7.Selection Naturalselection
Naturalselectionandevolution
Artificialselection
Artificialselectionandgenetics
8.Generalprinciplesofreproduction
Reproductionensuresthesurvivalofaspecies
36
36
9.Sexualreproduction
Malesandfemales
Meiosis
Fertilisation 38
38
40
42
Solutionstoactivities
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
lExplain 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
lExplain 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
Thecell
LearningOutcome2: Constructing science knowledge
AssessmentStandards
Recalling meaningful information when needed
Applying knowledge to problems that are not taught explicitly
LearningOutcome1: Scientific investigation
AssessmentStandards
Conducting investigations and collecting data
Evaluating data and communicating findings
Knowledgearea: Life and Living
Substrand: Biodiversity, change and continuity
Contenttopic: 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
Part1:UnderstandingGenesandInheritance
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
Thenucleus
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.
Part1:UnderstandingGenesandInheritance
HSRC Workbook
2 M
easuringverysmall structures
LearningOutcome2: Constructing science knowledge
AssessmentStandards
Recalling meaningful information when needed
Applying knowledge to problems that are not taught explicitly
LearningOutcome1: Scientific investigation
AssessmentStandards
Conducting investigations and collecting data
Evaluating data and communicating findings
Knowledgearea: Life and Living
Substrand: Biodiversity, change and continuity
Contenttopic: 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.
CrossingOver:TheBasicsofEvolution
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.
10
Part 1: Understanding Genes and Inheritance
HSRC Workbook
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.
12
Part 1: Understanding Genes and Inheritance
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 Thecellcycle
LearningOutcome2: Constructing science knowledge
AssessmentStandards
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
Knowledgearea:LifeandLiving
Substrand: Biodiversity, change and continuity
Contenttopic: 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?
Cellsdivide
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]
\gdli]
on\diZ
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.
\gdli]
Y^k^h^dc
\gdli]
6
7
\gdli]
\gdli]
Y^k^h^dc
\gdli]
Part1:UnderstandingGenesandInheritance
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
]
]d
gh
jg
dj
\gdli]>
Cell division
cZlbZbWgVcZ
Y^k^YZhXZaa
gZea^XVi^dc
e]VhZ
\gdli]>>
] d j gh
6
7
b^idh^h
] d jgh
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 hechromosomesandcell
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.
LearningOutcome2: Constructing science knowledge
AssessmentStandards
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
Knowledgearea:Life and Living
Substrand: Biodiversity, change and continuity.
Contenttopic: 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?
16
Part1:UnderstandingGenesandInheritance
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.
Part1:UnderstandingGenesandInheritance
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.
LearningOutcome2: Constructing science knowledge
AssessmentStandards
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
Knowledgearea: Life and Living
Substrand: Biodiversity, change and continuity
Contenttopic: 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.
Stepsintheprocess
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.
CrossingOver:TheBasicsofEvolution
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?
22
Part1:UnderstandingGenesandInheritance
HSRC Workbook
6 Inheritance Learningoutcome2: Constructing science knowledge
AssessmentStandards
Applying knowledge to problems that are not taught explicitly
Interpreting information
Recalling meaningful information when needed
Learningoutcome1:Scientific investigation
AssessmentStandard
Conducting investigations and collecting data
Knowledgearea: 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.
Parentsandoffspring
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,identicalandvariable
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.
CrossingOver:TheBasicsofEvolution
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.
24
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?
26
Part 1: Understanding Genes and Inheritance
HSRC Workbook
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
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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.
28
Part 1: Understanding Genes and Inheritance
HSRC Workbook
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?
30
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 Learningoutcome2:Constructing science knowledge
Assessment standard
Interpreting information
Learningoutcome3: Science, society and the environment
Assessmentstandard
Understanding science and technology in the context of history and indigenous knowledge
Knowledgearea: Life and Living
Substrand: Biodiversity, change and continuity.
Contenttopics: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.
Naturalselection
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,
32
Part1:UnderstandingGenesandInheritance
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.
34
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
eneralprinciplesof reproduction
Learningoutcome2: Constructing science knowledge
AssessmentStandards
Applying knowledge to problems that are not taught explicitly
Interpreting information
Recalling meaningful information when needed
Learningoutcome1: Scientific investigation
AssessmentStandards
Planning investigations
Conducting investigations and collecting data
Evaluating data and communicating findings
Knowledgearea: Life and Living
Substrand: Biodiversity, change and continuity
Contenttopic: 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
Contenttopic: Human reproduction begins with the fusion of sex cells from mother and
father, carrying the patterns for some characteristics of each.
Reproductionensuresthesurvivalofaspecies
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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Part1:UnderstandingGenesandInheritance
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 Sexualreproduction
Learningoutcome2: Constructing science knowledge
AssessmentStandards
Applying knowledge to problems that are not taught explicitly
Interpreting information
Recalling meaningful information when needed
Learningoutcome1: Scientific investigation
AssessmentStandards
Planning investigations
Conducting investigations and collecting data
Evaluating data and communicating findings
Knowledgearea:Life and Living
Substrand: Biodiversity, change and continuity.
Contenttopic: 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
Contenttopic: 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.
Malesandfemales
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.
Part1:UnderstandingGenesandInheritance
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? ________
40
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
45
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
46
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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9
11
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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
49
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.
50
Part 1: Understanding Genes and Inheritance