2. Gamete Formation Gametes are sex cells (sperm, eggs) Arise from germ cells testes ovaries anther ovary Figure 10.2a Page 164

3. Meiosis A special type of cell division Used to make sex cells (GAMETES) Meiosis halves the numbers of chromosomes Meiosis picks one chromosome from each pair at random and places them in a sex cell. This results in enormous variation amongst the sex cells. © 2007 Paul Billiet ODWSODWS

4. Chromosome Number Sum total of chromosomes in a cell Germ cells are diploid (2n) Gametes are haploid (n) Meiosis halves chromosome number

5. A process of reductional division in which the number of chromosomes per cell is cut in half. The word "meiosis" comes from the Greek verb meioun, meaning "to make small,"

6. Meiosis: Two Divisions Two consecutive nuclear divisions Meiosis I Meiosis II DNA is not duplicated between divisions Four haploid nuclei form

8. Meiosis I - Stages Prophase IMetaphase IAnaphase ITelophase I

9. Prophase I Chromosomes begin to condense. Cell is 2n (diploid) and 2 chromatids/chromosomes (like before mitosis). Synapsis occurs: homologous chromosomes pair, their DNA aligns. Each duplicated chromosome pairs with homologue Homologues swap segments, CROSSING OVER Each chromosome becomes attached to spindle

10. Metaphase I Homologous chromosomes align at cell equator to form tetrads. The spindle is fully formed

11. Homologous chromosomes seperate The sister chromatids remain attached

12. Telophase I The chromosomes arrive at opposite poles Usually followed by cytoplasmic division

13. Two daughter cells are formed.

14. Prophase II Microtubules attach to the kinetochores of the duplicated chromosomes

15. Metaphase II Duplicated chromosomes line up at the spindle equator, midway between the poles

16. Anaphase II Sister chromatids separate to become independent chromosomes

17. Telophase II The chromosomes arrive at opposite ends of the cell A nuclear envelope forms around each set of chromosomes Four haploid cells

18. After cytokinesis, four daughter cells are formed.

19. Crossing Over Each chromosome becomes zippered to its homologue All four chromatids are closely aligned Nonsister chromosomes exchange segments Figure 10.5 Page 168

20. Effect of Crossing Over After crossing over, each chromosome contains both maternal and paternal segments Creates new allele combinations in offspring

21. Possible Chromosome Combinations or 123 Figure 10.6 Page 169

24. 23 unpaired chromosomes Fertilisation Child Father 23 pairs of chromosomes Sex cells Meiosis Mother 23 pairs of chromosomes Meiosis and fertilisation © 2007 Paul Billiet ODWSODWS Images believed to be in the Public Domain

25. Comparing Meiosis and Mitosis Chromosome behavior Mitosis: Homologous chromosomes independent Meiosis: Homologous chromosomes pair forming bivalents until anaphase I Chromosome number Mitosis- daughter cells diploid Meiosis- daughter cells haploid Genetic identity of progeny: Mitosis: identical to parent cell Meiosis: not identical to parents cell