1. CONTENT Meiosis

2. LEARNING OUTCOMES Identify, with the aid of diagrams, the main stages of meiosis State how meiosis and fertilization can lead to variation

4. Meiosis Meiosis is essentially a two stage reduction cell division during which the chromosome number is reduced to half (n). Two divisions: meiosis I and meiosis II Like mitosis, both divisions are continuous processes, conveniently divided into PMAT(I) in meiosis I and PMAT (II) in meiosis II

5. The Cell Cycle (Interphase) During interphase, cell prepares for mitosis. It can be divided into three phases, G1 phase, S phase and G2 phase. Same as in interphase taking place before mitosis

6. centromere DNA replication in S phase of interphase Condensation or spiralisation in prophase Exist as chromatin threads; Each has a genetically identical, newly synthesized DNA chain Exist as chromatin threads A pair of homologous chromosomes

7. A pair of homologous chromosomes that have undergone DNA replication. Centromere Sister chromatids

9. Prophase I (Crossing Over) Can have more than 1 chiasma (Plural: chiasmata)

10. Prophase I (Crossing Over) Crossing over allow the exchange of genetic material between homologous chromosomes leading to new combination of alleles – genetic variation introduced) alleles are exchanges and thus the sister chromatids of the homologous chromosome are no longer identical.

11. Prophase I By the end of prophase 1, exchange of genetic material has occurred. Just like prophase of mitosis, the nucleolus and nuclear membrane will breakdown at the end of prophase 1.

12. Metaphase I The bivalents / homologous chromosomes move to the equator of the spindle. Arranged in such a way that centromeres of the two homologous chromosomes orientate towards opposite poles and attach to spindle fibres (form 2 rows) The orientation of maternal and paternal chromosomes towards the two poles is random* (independent assortment of chromosomes)

13. Independent Assortment of Chromosomes in metaphase I (further introduces genetic variation)

14. Metaphase I

15. Anaphase I Separation of the homologouschromosomes. Centromere-to-pole spindle fibres shorten and pull the centromere and the two sister chromatids that are attached to the centromeres towards opposite poles of the spindle.

16. Anaphase I Centromeres do not divide here. Sister chromatids remain attached to centromere as homologous chromosomes move to opposite poles. Number of chromosomes at each pole of the cell has been reduced by half, or from diploid (2n) to haploid (n) after forming back nuclear envelope during telophase I.

17. Anaphase I

18. Telophase I Arrival of chromosomes (consist of sister chromatids attached to same centromere) at respective poles Spindle fibres disintegrate. Nuclear envelope and nucleoli reform. Each daughter nucleus has haploid (n) set of chromosomes.

19. Note: In animals and some plants, the chromatids usually uncoil, a nuclear envelope forms round the chromosomes at each pole and the nucleus enters interphase again. But there is no replication of DNA.

20. Meiosis II In all the important ways, meiosis II is identical to mitosis. Example: In anaphase II, sister chromatids will separate instead of homologous chromosomes.

21. Prophase II The main events in this phase are similar to prophase of mitosis ie. Condensation of chromatin threads Formation of spindle fibres Breakdown of nuclear membrane and nucleolus.

22. Prophase II

23. Metaphase II Chromosomes will line up in a single row at the equator of spindle. Centromere of each chromosome is attached to spindle fibres.

24. Metaphase II

25. Anaphase II Centromeres divide Sister chromatids separate and pulled apart to opposite poles by spindle fibre. Once separated, sister chromatids with their own centromeres are referred to as chromosomes.

26. Anaphase II

28. Spindle fibres disintegrate.

30. Significance of Meiosis In Sexual Reproduction forms haploidgametes, for sexual reproduction. The nuclei of the haploid male and female gametes fuse during fertilization to produce a zygote with a diploid number of chromosomes. By halving the chromosome number, meiosis ensures that there is no doubling of chromosome number for each successive sexually reproduced generation.

32. It should also be able to colonize a range of new environment.

34. Genetic variations are generated through: (i)Mendel’s Law of Independent assortment Results in random / independentassortment of the maternal and paternal chromosomes during metaphase I and subsequent separation of homologous chromosomes during anaphase I. Number of possible maternal-paternal combinations = 2 to the power of n (n= no of homologous pairs) In humans, n = 23, no. of combinations = 8388608 Results in different combinations of chromosomes in daughter cells (some maternal and some paternal traits)

35. Independent assortment of Chromosomes

36. Genetic variations are generated through: (ii) Chiasmata formation and crossing over Crossing over of segments between non-sister chromatids of homologous chromosomes during prophase I of Meiosis I Leads to new combinations of alleleson chromosomes of the gametes.

37. Genetic variations are generated through: (ii) Random fusion of gametes E.g. No. of possible gametes formed in humans: 8388608 No. of possible zygote = 8 million X 8 million

38. Abnormal Events in Meiosis Lead to Changes in chromosome number Arise from meiosis due to non-disjunction Non-disjunction is the failure of the two homologous chromosomes of a pair to separate to the opposite poles at meiosis I, so that one daughter cell has both chromosomes and the other neither. Fusion of gametes results in abnormal chromosome number (e.g. 3 copies of chromosome 21 in Down’s Syndrome)

39. Comparison between Mitosis and Meiosis Please refer to notes for similarities and differences http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter12/animations.html The End 