1. Meiosis Producing sex cells

2. Definitions… Haploid – describes cells that have 1 copy of each chromosome (eg. gametes) Diploid – describes cells that have 2 copies of each chromosome (eg. Somatic cells) Gametes – sex cells Somatic cells – body cells (not sex cells eg. sperm and ovum)

3. How many chromosomes? Diploid cells Nucleus contains 46 chromosomes OvumC=23 SpermC=23

4. How do we make haploid cells? Haploid cells are produced by a special type of cell division called meiosis

5. Chromosomes Of the 46 chromosomes, 2 determine the sex of the organism. These are called sex chromosomes (XX = F; XY = M) The other 44 are called autosomes This cell would has 23 pairs of chromosomes (2 copies of each)

6. A Closer Look at Meiosis Meiosis involves 2 stages of division Stage I - like mitosis produces 2 diploid daughter cells Stage II – produces 4 haploid daughter cells

7. A Closer Look at Meiosis Interphase I Interphase I Prophase I Prophase I Metaphase I Metaphase I Anaphase I Anaphase I Telophase I Telophase I Note that there is no Interphase II Note that there is no Interphase II

8. Interphase Centromere Chromatid Back Interphase: The phase in a cell’s cycle when it is NOT dividing. This makes up most of the cell’s life- cycle. Towards the end of this phase, DNA replication occurs

9. Prophase Back Prophase: Early: The DNA is tightly wound up and shortened to form chromosomes. This prevents the DNA from becoming tangled. Late: The nuclear membrane breaks down and the ‘mitotic spindle’ forms starting from the centriole of the cell, causing the chromosomes to be pulled the middle area of the cell. Early Late

10. Metaphase Back Metaphase: During this phase the chromosome become aligned at the exact centre of the cell, held by the mitotic spindle.

11. Anaphase Back Anaphase: The chromosomes split at the centromere and the chromatids move towards opposite poles of the cell

12. Telophase Back Telophase: Once the chromosomes stop moving, a nuclear membrane begins to form and the mitotic spindle breaks up. A cell membrane forms around each new nucleus. Two new ‘daughter cells’ have been born.

13. Thus Sperm and Ova are formed Meiosis is the process of sperm and ova production The haploid products formed are sperm and ova But how come we don’t all look exactly like our brothers and sisters?

14. A Closer Look at Meiosis Metaphase I: In the first division, during metaphase, the homologous chromosomes undergo a process called ‘crossing over’ or genetic recombination. This involves the swapping of genetic material between homologous chromosomes

15. Metaphase I Metaphase I: In the first division, during metaphase, the homologous chromosomes undergo a process called ‘crossing over’ or genetic recombination. This involves the swapping of genetic material between homologous chromosomes

16. Metaphase I These links are called chiasmata (chiasma in singular)

18. Metaphase I Back Genetic material from the homologous chromosomes is randomly swapped This creates four unique chromatids Since each chromatid is unique, the overall genetic diversity of the gametes is greatly increased For more info visit: www.uic.edu/.../lecturesf04am