1. THE PROCESS OF MEIOSIS, MEIOSIS AND REPRODUCTION, MEIOSIS AND GENETICS Grade 12 Life Sciences Revision
Refer to the CAPS document and to
MEIOSIS, HUMAN REPRODUCTION AND GENETICS webpages

2. Introduction Review the structure of a cell (Grade 10) (E.G)
State that:
Chromosomes consists of DNA (which makes up genes) and protein
The number of chromosomes in a cell is a characteristic of an organism (for example humans have 46 chromosomes)
Chromosomes which are single stranded become
double stranded (two chromatids joined by a
centromere) as a result of DNA replication. You should
be able to draw and label a diagram of a chromosome.

3. Differentiate between: Haploid(n) and diploid(2n) cells in terms of chromosome number and sex cells (gametes) and somatic cells (body cells)
Differentiate between:
Sex chromosomes (gonosomes) and
autosomes

4. Meiosis – The process Define meiosis. (E. G)
a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores.
State where meiosis takes place in plants
and in animals.

5. Interphase
State that interphase takes place before meiosis and that although meiosis is a continuous process, the events are divided into different phases for convenience
Describe what happens during interphase as follows:
DNA replication takes place
Single stranded chromosomes become double-stranded
Each chromosome will now consist of two chromatids joined by a centromere

6. Describe the events of each phase of Meiosis I as follows:
Prophase I
Centrosome splits into centrioles which move to opposite poles
forming spindle threads.
Nuclear membrane and nucleus start to disappear.
Chromosomes become distinct.
Crossing over takes place as follows:
Homologous chromosomes lie next to each other
Non-sister chromatids of homologous chromosomes overlap
Point of overlap is called the chiasmata
A break occurs at the chiasmata
Chromatid segments (genetic material) have now been exchanged

7. MEIOSIS 1 – BEGINNING OF PROPHASE 1
Chromosomes are visible as doubles eg 2 chromatids joined together by a centromere
AND
each chromosome pairs up with one of the same size to form a BIVALENT

8. PROPHASE 1 maternal and paternal chromosomes exchange information

9. END OF PROPHASE 1 2N
Prophase 1
Look at evidence here that crossing over has taken place.
2 chromosome pairs visible

10. Meiosis 1 continued Metaphase I
Chromosomes align at equator in a random fashion and in homologous pairs (bivalents), attached to the spindle threads on the same fibre, using centromeres.
Anaphase I
Spindle threads contract.
Homologous chromosomes are separated and move apart towards the poles.
The cytoplasm starts to divide.
Telophase I
2 new cells have been formed.
Each new cell has half of the chromosome complement of the original cell (it is haploid).
The cells are not identical.

11. METAPHASE 1 2N
Metaphase 1
Bivalents line up on the equator and are attached to the spindle fibres by centromeres

12. ANAPHASE 1
Anaphase 1
Spindle fibres contract towards poles pulling whole chromosomes apart

13. TELOPHASE 1
TWO cells formed with too much information – still need to halve the amount of DNA

14. Describe the events of each phase of Meiosis II as follows:
Prophase II
Centrosome splits into centrioles which move to opposite poles forming spindle threads/fibres.
Metaphase II
Chromosomes align at equator in a random fashion and in a single row attached to the spindle threads by centromere.
Anaphase II
Spindle threads contract.
Centromere of each chromosome splits.
Chromatids of each chromosome move apart towards the opposite poles.
The cytoplasm starts to divide.
Telophase II
4 new cells have been formed.
Each new cell has half of the chromosome complement of the original cell (it is haploid).
The cells are not identical

15. TELOPHASE 1
TWO cells formed with too much information – still need to halve the amount of DNA

16. PROPHASE 2 THIS HAPPENS IN BOTH CELLS AT THE SAME TIME
2 separate chromosomes are visible in each cell

17. METAPHASE 2
Metaphase 2
Chromosomes align individually on the equator and attach to spindle by centromeres

18. ANAPHASE 2
Anaphase 2
Spindle fibres contract pulling chromatids apart

19. TELOPHASE 2 AND CYTOKINESIS
NOTE EACH DAUGHTER CELL HAS TWO CHROMOSOMES

20. Importance of Meiosis
State the importance of meiosis with regard to each of the following:
Production of gametes (refer to detail of gametogenesis)
Halving of the chromosome number (diploid to haploid)
Mechanism to introduce genetic variation through:
Crossing-over

21. Crossing over

22. More on crossing over
Mechanism to introduce genetic variation through:
Crossing-over

23. The random arrangement of chromosomes at the equator

24. Abnormal Meiosis State what is meant by non-disjunction
State the consequences of non-disjunction
Describe how non-disjunction of chromosome pair 21 in humans may lead to the formation of an abnormal gamete with an extra copy of chromosome 21
Describe how the fusion between the abnormal gamete and a normal gamete may lead to Down syndrome

25. Normal Meiosis

26. Non- disjunction
Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division

28. Cell division and no - disjunction
There are three forms of nondisjunction:
failure of a pair of homologous chromosomes to separate in meiosis I,
failure of sister chromatids to separate during meiosis II,
and failure of sister chromatids to separate during mitosis.
Nondisjunction results in daughter cells with abnormal chromosome numbers (aneuploidy).

29. Down syndrome
Down's syndrome is caused by the presence of three copies of the 21st chromosome.
This chromosomal defect is known as Trisomy-21.
Down's syndrome almost always results in mental retardation, though the severity of the retardation varies.

30. Characteristics of Down syndrome
Small, oblique eyes
Flattened, nasal bridge
Open mouth Protruding tongue Broad neck
Small underdeveloped ears set low on head
Epicanthic fold

31. Incurved finger
Single palmar (“simian”) crease
Short broad hands
Wide gap between first and second toe

32. Comparison of mitosis and meiosis
How are mitosis and meiosis similar? 
Both mitosis and meiosis are associated with cytokinesis. The end result of both are daughter cells produced from a parent cell.
The fundamental sequence of events in mitosis is the same as in meiosis (in meiosis it happens twice).
Both processes include the breakdown of the nuclear membrane, the separation of genetic material into two groups, followed by cell division and the reformation of the nuclear membrane in each cells

33. Tabulate differences between mitosis and meiosis

34. Meiosis and sexual reproduction 1
Relationship between Gametogenesis and Meiosis
Define each of the following terms:
Gametogenesis
Spermatogenesis
Oogenesis

35. Meiosis and sexual reproduction 2
Describe spermatogenesis as follows: Refer to cross section of seminiferous tubule and diagram of a sperm cell
Diploid cells in the seminiferous tubules of the testes undergo meiosis
to form haploid sperm cells
Identify and state the functions of the parts of a sperm cell (acrosome, head with haploid nucleus, middle  portion with mitochondria, and a tail)

36. Meiosis and sexual reproduction 3
Describe oogenesis as follows: Refer to longitudinal section of the ovary and diagram of an ovum
Diploid cells in the ovary undergo meiosis to form a primary follicle
consisting of haploid cells
One cell develops into an ovum contained in a Graafian follicle
Identify and state the functions of the different parts of an ovum (layer of jelly, haploid nucleus, cytoplasm)

37. Meiosis and genetics 1 FROM THE CAPS DOCUMENT:
State Mendel’s principle of segregation
‘Each pair of alleles is sorted into different gametes and
subsequently into different offspring. This is the result
of the way each allele is carried on separate
homologous chromosomes that are separated during
meiosis’.
This is important to remember when doing
monohybrid and dihybrid crosses.

38. Meiosis and genetics 2 Consequences of abnormal meiosis
If meiosis does not proceed according to plan the resulting gametes may have an uneven number of chromosomes. This is usually caused by non-disjunction of chromosomes during Anaphase 1. The most well known example of non-disjunction is Trisomy-21 or Down Syndrome. This is a form of chromosomal aberration rather than a mutation. Refer to the karyotype of this person with Trisomy-21. The person is diploid but has one extra chromosome or three of the number 21 chromosomes.

39. Sources of information
Biozone Power Point ‘ Inheritance PP’
Available at: