A Powerpoint for Grade 12 Life Sciences / Biology students focussing on chromosomes and meiosis. Contains information and diagrams on meiosis, mitosis, the structure of chromosomes, DNA and RNA

1. Module Three: Life at a Molecular, Cellular and Tissue Level
Life Sciences
Matric Syllabus
Mind Action Series: Life Sciences
Textbook and Workbook

2. • A bivalent is a pair of homologous chromosomes in close
contact with each other.
• Chromosomes are long, thread-like structures that form part
of the chromatin network in the nuclei of cells, made up of a
strand of DNA wound around a histone (protein).
• Chiasmata are the points of crossing over where the
chromatids break.
3.2) Chromosomes and Meiosis

3. • Chromosomes are long, thread-like structures that form part
of the chromatin network in the nuclei of cells.
• They consist of a strand of DNA wound around histones
(proteins).
• A set of chromosomes in a cell is called a karyotype. It shows
the number, size and shape of the chromosomes during the
metaphase of mitosis.
• They are useful as they can show whether a cell comes from a male
or female, as well as any abnormalities in the chromosomes.
• Every species has a specific number of chromosomes in its
somatic cells. Some organisms have identical chromosome
numbers but these need not be related.
3.2) Chromosomes and Meiosis

4. • In somatic (body) cells of diploid organisms:
– The number of chromosomes in each cell is the same.
– Chromosomes are made up of two sets: one from the mother, one
from the father. They are called diploid cells, or 2n.
– A maternal chromosome will have a matching paternal chromosome.
Together they will form a homologous pair. The chromosomes
forming a pair will have the same size and shape, but may have
different alleles for each trait.
– The DNA of each chromosome replicates to form two identical
threads or chromatids joined by a centromere. This takes place in the
interphase of a cell cycle, i.e. between cell divisions.
– Replication of DNA is very important to ensure that, as a cell divides,
each daughter cell receives a full complement of all the genetic
material.
3.2) Chromosomes and Meiosis

5. 3.2) Chromosomes and Meiosis

6. • Meiosis is cell division that takes place in the reproductive organs
of both plants and animals to produce gametes (sex cells) in
animals and spores in plants.
• In meiosis, the number of chromosomes is reduced from two sets
(2n) to one set (n) in each new daughter cell.
• The gametes/spores are called haploid cells because they only
have one set of chromosomes, i.e. one chromosome from each
homologous pair.
• In animals, meiosis takes place in the reproductive organs, the
testes (spermatogenesis) and ovaries (oogenesis).
• In plants, meiosis takes place in the male anthers to form pollen
sacs (microsporangia) and in the female ovaries to form ovules
(megasporangia).
3.2) Chromosomes and Meiosis

7. 3.2) Chromosomes and Meiosis

8. 3.2) Chromosomes and Meiosis

9. • The DNA of the parent cells is replicated in interphase
preceding both meiosis and mitosis. However, in meiosis,
replication is followed by two divisions.
– Meiosis 1 is a reduction division which results in two cells being
formed, each with half the number of chromosomes of the parent cell,
i.e. the haploid number (n).
– Meiosis 2 is a copying division which involves the two haploid cells
dividing again by mitosis to form 4 haploid cells.
3.2) Chromosomes and Meiosis

10. • In early prophase, chromosomes become short, fat, and visible.
• In late prophase, the chromosomes of homologous pairs lie along
side one another, forming a bivalent. The centrioles move to opposite
poles. A spindle, made of protein threads, develops across the cell
from the two centrioles. At this point, crossing over takes place.
Nuclear membrane breaks down.
3.2) Chromosomes and Meiosis

11. • The bivalents (not the chromosomes) move to the middle of
the cell and line up at the equator.
• The centromeres become attached to the spindle threads.
3.2) Chromosomes and Meiosis

12. • The centromeres do not split. The bivalents separate and the
chromosomes, not the chromatids, are pulled away from each
other by the contracting spindle threads. The chromosomes
move to opposite poles of the cell.
3.2) Chromosomes and Meiosis

13. • The cytoplasm then divides via cytokinesis to form two haploid
cells. Both the new cells only have one of each homologous pair of
chromosomes.
3.2) Chromosomes and Meiosis

14. • Each chromosome is made up of two chromatids joined by a
centromere. The spindle, made up of protein fibres develops.
The nuclear membrane disappears.
3.2) Chromosomes and Meiosis

15. • Chromosomes move to the middle of the cell where they line up at
the equator. The centromeres become linked to the spindle threads.
3.2) Chromosomes and Meiosis

16. • The centromeres split, allowing each chromosome to separate into
two chromatids. Spindle threads contract and pull the chromatids
apart. The chromatids, now called daughter chromosomes, move to
the poles of the cell.
3.2) Chromosomes and Meiosis

17. • Daughter chromosomes group together at the poles. A new
nuclear membrane starts to form around each set of daughter
chromosomes.
3.2) Chromosomes and Meiosis

18. • The cytoplasm starts to divide forming two new daughter cells, each
with the haploid number of chromosomes. A new nucleolus forms.
3.2) Chromosomes and Meiosis

19. • Meiosis 1
– A reduction division
– Early Prophase  Late Prophase  Metaphase  Anaphase 
Telophase
• Meiosis 2
– A copying division
– Late Prophase  Metaphase  Anaphase  Telophase  Cytokinesis
• At the end of meiosis four new, non-identical, haploid cells are
formed from one parent cell, each with half the original
number of chromosomes. The gametes are not identical to
the parent cell.
3.2) Chromosomes and Meiosis

20. 1. First meiotic division
Homologous chromosomes come
together to form a bivalent; one
1 from each pair goes into each
daughter cell
2. Two haploid daughter cells
2
3. Second meiotic division
Each chromosome separates into
3 two chromatids, one goes into each
daughter cell
4. Four haploid daughter cells
4
3.2) Chromosomes and Meiosis

21. • The number of chromosomes has to be halved – or on
fertilization, the zygote would have double the number of
chromosomes. The next generation will have double the
number of chromosomes, and so on.
• Meiosis makes new gene combinations come about, resulting
in variation of offspring.
3.2) Chromosomes and Meiosis

22. • Crossing over is the mutual exchange of pieces of chromosome so
that whole groups of genes are swapped between maternal and
paternal chromosomes. This takes place in the late prophase of
meiosis 1.
• The replicated homologous pairs of chromosomes come together in
a process called synapsis to form bivalents. They swap pieces of
their inner chromatids by breaking and reforming their DNA while
they are paired up.
• The points of crossing over where the chromatids break are called
chiasmata.
• In this way, some genes from a maternal chromatid change place
with some genes from a paternal chromatid, forming a recombinant
chromatid. The outer, unchanged chromatids are called parentals.
3.2) Chromosomes and Meiosis

23. 3.2) Chromosomes and Meiosis

24. • The exchange of genetic material produces chromatids with a
unique combination of genes. This increases variation among
the daughter cells as there will be new combinations of
genetic material. This is why offspring will not look the same
(except for identical twins) or the same as one parent.
• During this exchange, mistakes may occur which lead to
mutations. Most mutations are harmful but occasionally may
be beneficial. In this way, new genes may be introduced into
the genetic make up of a species which can influence
evolution.
3.2) Chromosomes and Meiosis

25. Similarities:
• Both are types of cell division.
• The DNA of the parent cells is replicated in interphase before
cell division starts.
• In early prophase, the chromosomes become short and fat,
and are visible as two chromatids joined by a centromere.
3.2) Chromosomes and Meiosis

26. Process Mitosis Meiosis
Purpose - Development of an adult organism - Forms gametes or spores (for
from a single zygote reproductive purposes)
- Growth and repair of tissues
- Regeneration of body parts
- Asexual reproduction
Differences Involves ONE cell division Involves two cell divisions
In prophase, no bivalents are formed and In prophase, bivalents are formed and
no crossing over occurs. crossing over occurs
In metaphase, the centromeres split In metaphase, centromeres do not
split
In anaphase, the chromatids of each In anaphase, the chromosomes of
chromosome move to opposite poles of each homologous pair move to
the cell. opposite poles of the cell.
Two daughter cells are formed with the Four daughter cells formed with half
same number of chromosomes as the the number of chromosomes as the
parent cell (diploid) parent cell (haploid)
Somatic cells are formed which are Gametes are formed which are
similar genetically to the parent. genetically different to each other and
to the parent cell.
Location Takes place in growing regions of Takes place in reproductive organs
plants/animals

27. • ‘Genetic variation’ refers to the differences which exist between
organisms belonging to the same species
• In organisms which reproduce sexually, every one of the offspring
(excepting for identical twins) possess a unique combination of
genes, therefore there is always variation in sexual reproduction.
3.2) Chromosomes and Meiosis

28. • This variation of the offspring is because their genotypes differ as a
result of:
– The crossing over of pieces of chromatids which causes a reshuffling of genes
in the gametes formed.
– The random movement of maternal and paternal chromosomes to opposite
poles of the cell during anaphase of meiosis. This is called the independent
assortment of chromosomes and it results in every egg and every sperm
formed containing a mixture of maternal and paternal chromosomes, i.e. a
huge variety of genetic combinations.
– The sheer chance as to which particular sperm fertilises an egg cell during
fertilisation. Any two gametes can fuse, resulting in many possible
combinations of genes in the zygote. The new individual will have a unique
combination of genes, different from either of its parents and siblings.
– Mutations.
3.2) Chromosomes and Meiosis

29. • A mutation is a sudden and unpredictable change in the
genetic makeup of an organism. This may be caused by a:
– Gene mutation
– Chromosome mutation. This type of mutation can only occur during
meiosis. It involves a change in the structure and distribution of one or
more chromosomes and therefore results in a change in the cell’s
karyotype.
3.2) Chromosomes and Meiosis

30. • Polyploidy is a type of chromosome mutation.
• It is a condition of having more than two sets of
chromosomes. It is rare in animals but is especially common
among ferns and flowering plants. GM’d wheat has strains
such as durum wheat that are tetraploid (4n) and bread
wheat that are hexaploid (6n).
• Polyploidy can occur:
– Naturally in certain plant tissues
– As a result of abnormal meiosis. If the chromosomes do not split
during anaphase 1, the gametes will be diploid and when these fuse a
tetraploid zygote will occur.
3.2) Chromosomes and Meiosis

31. Advantages to Polyploidy:
• Creates an instant new species, therefore it plays an
important role in evolution.
• Polyploid plants will have:
– Larger fruits
– Larger flowers
– Larger storage organs.
3.2) Chromosomes and Meiosis

32. • Anaploidy is another type of chromosome mutation where
cells have extra chromosomes or missing chromosomes.
• Affected individuals will have mental and physical
characteristics called syndromes.
• One such example is Down Syndrome.
3.2) Chromosomes and Meiosis

33. Down syndrome is an example of anaploidy that occurs in children
who are born with an extra copy of chromosome number 21 in
their cells (i.e 2n + 1), a condition known as trisonomy.
How Down Syndrome Occurs:
• During oogenesis (meiosis in the production of an egg), the two
number 21 chromosomes do not separate properly in anaphase
one.
• Both chromosomes enter one daughter cell, instead of one
entering each of the daughter cells formed.
• The zygote will have three number 21 chromosomes instead of
two, and a total of 47 chromosomes instead of 46.
• As the new embryo develops by mitosis, all the cells will have 47
chromosomes.
3.2) Chromosomes and Meiosis

34. Characteristics of those affected by Down Syndrome:
• Varying degrees of mental retardation, differing from person
to person.
• Distinctive flattened facial features with slightly slanting eyes
due to folds of skin at the corner of the eyes.
• Other physical features include short stubby fingers and toes
with big toes spaced widely apart from the second toe, a
large head and abnormal ears.
• Heart defects
• Happy, loving nature
There is no cure for down syndrome.
3.2) Chromosomes and Meiosis

35. • Down syndrome is relatively common – 1 in every 900 births are to
a Down syndrome child. The older the mother, the more likely
Down syndrome will occur.
• Down Syndrome children develop slower than others. Some Down
syndrome children can attend mainstream schools.
• Affected children need support as they are often discriminated
against.
• A test taken early in pregnancy, involving an ultrasound and a
blood test, can more or less pinpoint whether the foetus has Down
syndrome or not.
• If that test shows that the foetus may be affected by Down
Syndrome, the mother could choose to have a risky amniocentesis,
which determines the karyotype of the foetus.
3.2) Chromosomes and Meiosis