1. The production of male and female gametes
Meiosis
The production of male and female gametes

2. Why Meiosis?
1) Two cells, a sperm and an egg, unite to form a zygote, the single cell from which the organism develops
2) Meiosis is the process of producing sperm (in the testes) and eggs (in the ovaries) These are called gametes.
3) Meiosis ensures that there is the correct number of chromosomes present when sperm cells and egg cells unite.

3. Significance of meiosis
Leads to halving of chromosome number, so to ensure that the diploid number of chromosomes can be restored after fertilization

4. Why have sexual reproduction?
1) Shuffling of information between parents and offspring leads to new combinations. Bad combinations die without reproducing; good combinations survive and reproduce more offspring.

5. 2) Cells having pairs of homologous chromosomes - diploid (2n)
e.g. body (somatic) cells
46 chromosomes total in humans
3) Cells having one chromosome from each homologous pair - haploid (n)
e.g. gametes
23 chromosomes total in humans

6. Diploid and Haploid
Body Cell
Gamete

7. How many divisions? Two nuclear divisions
Meiosis I and meiosis II
Four haploid cells are produced
Occurrence of Meiosis:
Plants: anthers and ovules
Mammals: testes and ovaries

8. Prophase I
Chromosomes become visible
Nuclear membrane disappears

9. Prophase I Halves of chromosomes pair up and crossing over can occur.
92 chromosomes total = doubled from 46 in interphase.

10. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

11. Metaphase I
Homologous chromosomes line up at the middle of the cell randomly
This is called independent assortment
92 chromosomes

12. Independent assortment

13. Metaphase 1 Additions
1) Chromatids – when a chromosome replicates before meiosis two chromatids are produced. (sister chromatids)
2) Each pair of sister chromatids lines up with its homologous pair and joins together at their centromeres forming a tetrad or bivalent. This process is called synapsis

14. Anaphase I
Homologous chromosomes separate and are pulled to opposite ends.
92 chromosomes

15. Telophase I Nuclear membrane reforms and cytoplasm divides.
Each cell has half the chromosome number as the parent cell
92 chromosomes total = 46 in each cell.

16. Second meiotic division
Separation of chromatids of each chromosome
4 daughter cells with half of the chromosome number of the parent cells are formed

17. Prophase 2
Same as meiosis one, but with 2 cells, 46 in each cell.

18. Metaphase 2
Same as meiosis one, but with 2 cells, 46 chromosomes in each cell.

19. Anaphase 2
Same as meiosis one, but with 2 cells, 46 chromosomes in each cell.

20. Telophase 2
Same as meiosis one, but with 4 cells. 23 chromosomes in each cell.

21. Significance of meiosis
Produce genetic variation at
crossing-over between homologous chromosomes during prophase I
independent assortment of chromosomes during metaphase I

22. Sources of genetic variation
Crossing-over between homologous chromosomes during meiosis
Independent assortment of chromosomes during meiosis
Random fusion of gametes during fertilization
Mutation

23. Why do the members of this family look different?