UNIT 8 The Continuity of Life: Reproduction, Genetics and Evolution
Recommended Prior Knowledge
The major part of this Unit stands alone. Helpful, but not essential, would be a knowledge of cell structure, enzymes, nutrition and excretion and bacteria.
The general thread of reproduction runs throughout the Unit – first in general terms then, more specifically, in terms of genetic inheritance leading on to
variation and finally to evolution.
First, reproduction is considered in general terms, then sexual reproduction in both plants and animals is addressed. Sexually transmitted diseases are
studied together with their control. The importance of DNA is considered along with simple inheritance. A study of variation leads to an explanation of
evolution. The Unit ends with DNA function and genetic engineering.
N.B. Although reproduction and genetics are closely linked topics, since this is the longest of the Units, it may prove more convenient to separate
Reproduction from Genetics and Evolution, thus treating them as two separate Units.
Learning Outcomes
Suggested Teaching Activities
Online Resources
a) Define mitosis as cell division giving
rise to genetically identical cells in which
the chromosome number is maintained
and state the role of mitosis in growth,
repair of damaged tissues, replacement
of worn-out cells and asexual
Students will need to know that the nucleus
of a cell contains a number of
chromosomes and that the number is fixed
and constant for each species (46 in the
human being). A new body cell must be an
exact copy of the cell producing it. N.b.
Details of stages in mitosis are not
b) Define asexual reproduction as the
process resulting in the production of
genetically identical offspring from one
For the chosen commercial application
(e.g. a potato tuber) students should
understand the benefits and the
Other resources
‘GCE O Level Examinations
Past Papers with Answer
Guides (Biology)’ is produced
by CIE (Foundation Books).
CIE also produces the same
material on CD.
M. & G. Jones – 8
Mary Jones – Unit 18
Reproduction in Plants
Ian J. Burton – Topic 20
parent and describe one named,
commercially important application of
asexual reproduction in plants.
disadvantages of this method of
c) State that gametes are the result of a
reduction division in which the
chromosome number is halved from
diploid to haploid.
Details of meiotic division are not required
other than its halving of the chromosome
number. The terms gamete, diploid and
haploid should be explained.
d) Define sexual reproduction as the
process involving the fusion of haploid
nuclei to form a diploid zygote and the
production of genetically dissimilar
Learning Activity
Students should appreciate that each
parent therefore makes an equal
contribution to the diploid cell from which
an offspring will develop.
e) Identify and draw, using a hand lens if
necessary, the sepals, petals, stamens
and carpels of one, locally available,
named, insect-pollinated, dicotyledonous
flower, and examine the pollen grains
under a light microscope.
If possible, a large, brightly-coloured,
scented flower with visible nectar should be
chosen. Ensure that students produce
large drawings, with a sharp HB pencil,
draw clean lines and give the magnification
of their drawing (e.g. x3).
Learning Outcome
f) State the functions of the sepals, petals,
anthers and carpels.
Learning Activity
g) Use a hand lens to identify and
describe the anthers and stigmas of one,
locally available, named, wind-pollinated
flower, and examine the pollen grains
under a light microscope.
Learning Outcomes
It will be necessary to explain that carpels
are made up of component parts – stigma,
style, ovary and ovules. Also ensure that
students are clear that pollen (grains) are
not gametes but that they contain the
Note that a drawing is not required, but it
would be valuable for students to list any
noticeable differences from the features
seen in the insect-pollinated flower.
‘Teaching and Assessing
Practical Skills in Science’
Dave Hayward (CUP) pages
h) Outline the process of pollination and
distinguish between self-pollination and
Learning Activity
i) Compare, using fresh specimens, an
insect-pollinated and a wind-pollinated
Learning Outcome
j) Describe the growth of the pollen tube
and its entry into the ovule followed by
fertilisation (production of endosperm and
details of development are not required).
Learning Activity
k) Investigate and describe the structure of
a non-endospermic seed in terms of the
embryo (radicle, plumule and cotyledons)
and testa, protected by the pericarp (fruit
Learning Outcomes
l) State that seed and fruit dispersal by
wind and animals provides a means of
colonising new areas.
Learning Activity
Supply students with an example of a
wind- and of an animal-dispersed fruit or
Continuing the theme in c) and d) above,
the implications of self-pollination can be
Students should be invited to produce a list
of noticeable differences, but a definitive
table should then be provided for
An OHP transparency will offer the chance
to show the path taken by the pollen tube
and can be suitably labelled. On an
unlabelled version of the diagram, students
may then add the relevant detail.
Pea or large bean seeds are suitable,
soaked for 24 hours before use. This
investigation offers students the opportunity
for further drawing practice and, if time
permits, the chance to revise food tests on
substances stored in the cotyledons.
Students may be asked to suggest
advantages of the ability to colonise new
Stress that fruit and seed dispersal by wind
or animals can happen only after
pollination (by wind or insects) and the two
very different processes must not be
(many wind examples)
seed. Instruct students to construct a table
of differences between the two and
suggest reasons for the features they
observe. Ideally they should be those
considered in m) below.
Although there are adaptations for different
methods of animal dispersal, only one need
be considered in detail.
Learning Outcomes
m) Describe the external features of one,
locally available, named example of a
wind-dispersed fruit or seed and one
named example of an animal-dispersed
fruit or seed
Note that ‘warmth’ is scientifically vague
and that seeds surrounded by ‘moisture’ do
not germinate –atmospheric air contains
n) Investigate and state the environmental
conditions that affect germination of
seeds: suitable temperature, water and
For reasons of safety and expense, the
pyrogallol container might take the form of
one demonstration experiment set up by
the teacher. This activity reinforces the
need for a control in experimental work.
Learning Activity
Containers of seeds should be set up, one
lacking only a suitable temperature
(placed in ‘fridge at c. 4 oC), one lacking
only water and one lacking only oxygen
(sealed and containing alkaline pyrogallol).
Also a control, with seeds exposed to all
three conditions.
Learning Outcomes
o) Describe the uses of enzymes in the
germination of seeds.
This section echoes the work on enzymes
in Unit 3, 5 o) and links also to the work on
enzymes in Unit 1, 3. The need to convert
insoluble storage compounds into soluble
ones that can be transported should be
The remainder of this unit may generate
important, wider discussion which the
teacher should be prepared to answer both
honestly and sensitively.
Ian J. Burton – Topic 21
Sexual Reproduction in
Human Beings
Mary Jones – Unit 19
Reproduction in Humans
p) Identify on diagrams of the male
reproductive system and give the
functions of the testes, scrotum, sperm
ducts, prostate gland, urethra and penis.
q) Identify on diagrams of the female
reproductive system and give the
functions of the ovaries, oviducts, uterus,
cervix and vagina.
r) Compare male and female gametes in
terms of size, numbers and mobility.
s) Describe the menstrual cycle, with
reference to the alternation of
menstruation and ovulation, the natural
variation in its length and the fertile and
infertile phases of the cycle.
t) Describe fertilisation and early
development of the zygote simply in terms
of the formation of a ball of cells that
becomes implanted in the wall of the
u) State the function of the amniotic sac
and amniotic fluid.
v) Describe the function of the placenta
and umbilical cord in relation to
exchange of dissolved nutrients, gases
and excretory products (no structural
details are required).
w) Describe the special dietary needs of
p) and q) are outcomes that lend
themselves to the use of labelled OHP
transparencies for descriptive purposes
followed by an unlabelled versions of the
diagrams for students to label.
Comparisons should be supported by
reasons for the differences.
Annotated diagrams showing the cycle
divided into days and showing the build-up
and breakdown of the uterus lining are
The location of fertilisation should be
clearly described. Division of the zygote by
mitosis prior to implantation should be
Ways in which the embryo is protected by
the fluid (contained by the sac) should be
Links here with Unit 1,2 a), Unit 3, 5 a) c) n)
and s), Unit 4, 7 j) k) and Unit 5, 9 a) b) and
c). Stress that maternal and fetal bloods
do not mix.
The appropriate reason for each additional
dietary requirement should be mentioned.
As well as the benefits of breast milk,
deficiencies of bottle milk should be
menstrual cycle – details
(animation) and issues
amniotic fluid animation
pregnancy animation
vaginal delivery animation
(benefits of breast feeding)
pregnant women.
x) Describe the advantages of breast
milk compared with bottle milk.
Family planning clinics are often helpful in
supplying information and exhibits.
y) Describe the following methods of birth
control: natural, chemical (spermicides),
mechanical, hormonal and surgical.
The potentially severe nature of syphilis
should be mentioned, also that concern
about AIDS has diverted attention away
from other STDs. The need for early
treatment should be stressed.
z) Explain that syphilis is caused by a
bacterium that is transmitted during
sexual intercourse.
aa) Describe the symptoms, signs,
effects and treatment of syphilis.
Ensure that students are clearly aware that
no cure is yet available, but, with care, its
spread can be restricted.
b) Discuss the spread of human
immunodeficiency virus (HIV) and
methods by which it may be controlled.
a) Describe the difference between
continuous and discontinuous variation
and give examples of each.
Learning activity
Students can draw graphs to show the
distribution of height or weight within the
class. It might be insensitive, however, to
use these examples if there is a class
member who is particularly tall, short, fat
or thin. The variation in length of forearm
from elbow to fingertip might then be a
more thoughtful alternative.
Ian J. Burton – Topic 22
M. & G. Jones – 12
Inheritance and evolution
Body weight and height are standard
examples of continuous variation, and
blood groups or sex of discontinuous
variation. Graphs should be drawn of the
distributions shown by the two types of
Mary Jones – Unit 20
Learning Outcomes
b) State that a chromosome includes a
long molecule of DNA.
Detail of DNA structure is not required, but
later explanation becomes easier if it is
described as two long strands cross-linked
by a succession of paired molecules called
c) State that DNA is divided into sections
called genes.
Thus each gene comprises its own
particular sequence of linked bases.
d) Explain that genes may be copied and
passed on to the next generation.
When a cell divides, its nucleus first makes
an exact copy of each strand of DNA (and,
therefore, also of each gene). The original
passes into the nucleus of one cell, and its
copy passes into the other. In this way, the
same genes are passed from generation to
e) Define a gene as a unit of inheritance
and distinguish clearly between the terms
gene and allele.
f) Describe complete dominance using
the terms dominant, recessive,
phenotype and genotype.
g) Describe mutation as a change in the
structure of a gene (sickle cell anaemia)
or in the chromosome number (47 in
Down’s syndrome instead of 46).
(also, rev6.shtml & rev7.shtml)
Each gene represents one piece of
information – instructions for a certain
characteristic - passed from parent to
offspring. Alleles are different varieties of
the same gene.
These terms follow naturally from a
consideration of genes and alleles above. It
may be illustrated within the class by the
gene for tongue rolling, though a true
understanding of genotype is unlikely to be
gained until i) below.
Though genes are handed on from
generation to generation, they are subject
to change, causing a change in phenotype.
Most changes are very small and barely
noticeable, others have a more marked
effect. Change can also occur in
chromosome number.
h) Name radiation and chemicals as
factors that may increase the rate of
i) Predict the results of simple crosses
with expected ratios of 3:1 and 1:1, using
the terms homozygous, heterozygous, F1
generation and F2 generation.
j) Explain why observed ratios often differ
from expected ratios, especially when
there are small numbers of progeny.
k) Explain codominance by reference to
the inheritance of the ABO blood
phenotypes (A, B, AB, O, gene alleles IA,
IB and Io).
l) Describe the determination of sex in
humans (XX and XY chromosomes).
m) Describe variation and state that
competition leads to differential survival
of organisms, and reproduction by those
organisms best fitted to the environment.
Mutagens can have the effect of altering
the molecular structure of a gene and thus
altering the way in which the gene works.
Students should be encouraged to draw full
genetic diagrams to show these crosses.
The diagrams should be annotated and
include reference to parents and gametes.
It should be stressed that the ratios are
statistical and are those obtained only from
large samples. Such diagrams demonstrate
the same phenotype may have different
(but use the upper-case allele
superscripts specified in the
Students will observe that there can be
more than two alleles of the same gene.
It will be necessary to point out the sex
inheritance is the result of the inheritance
of chromosomes, not of genes.
The point needs to be made that variation
is a random process which happens to
leave some members at a survival
advantage over other organisms in a
particular environment. It does not occur ‘in
order to adapt to the environment’ as
students often believe. Organisms that
survive then reproduce and hand on the
advantage to at least some of their
n) Assess the importance of natural
selection as a possible mechanism for
Survival of the better-adapted organisms,
each themselves showing variations, some
of which are advantageous, generation
after generation, leads to evolutionary
change in the species.
o) Describe the role of artificial selection
in the production of economically
important plants and animals.
When humans select organisms with
characteristics that are commercially
desirable and breed from them, variation
can again enhance the selected
characteristic – especially over many
p) Explain that DNA controls the
production of proteins.
q) State that each gene controls the
production of one protein.
The linking of amino acids to form a
protein in the cells of a body is determined
by DNA. The exact amino acids and their
particular sequence in each different
protein are controlled by one specific gene.
r) Explain that genes may be transferred
between cells (reference should be made
to transfer between organisms of the
same or different species).
Human-to-human transfers in the case of
treatment for cystic fibrosis may be
explained as well as human to bacterium
and from (disease-resistant) plant species
to (non-resistant) plant species.
The insulin gene can be identified and, with
the aid of enzymes, isolated from the
strand of DNA in the nucleus of a cell from
a healthy person. Enzymes are again used
to attach it to the DNA of a bacterium.
Culture and extraction should be explained
on lines similar to Unit 7, 14 e).
s) Explain that the gene that controls the
production of human insulin can be
exerted into bacterial DNA.
t) Understand that such genetically
engineered bacteria can be used to
produce human insulin on a commercial
u) Discuss potential advantages and
dangers of genetic engineering.
then click on link to Gene
Transfer in Insulin Production
(under Biological Engineering)
(advantages and
Students may have strong ethical as well
as scientific views on this topic, but the
teacher should ensure that a balanced
consideration prevails.