1. Chromosomes & Meiosis

2. MAIN IDEAS You have body cells and gametes. Your cells have autosomes and sex chromosomes. Body cells are diploid; gametes are haploid

3. Cells can be divided into two major groups. Somatic Cells & Gametes 1.Body cells also called Somatic Cells Each human cell has 46 chromosomes or 23 pair. Diploid (2N) Diploid (2N)- cell has two copies of each chromosome: one from mom & one from dad

4. Each pair are Homologous chromosomes- meaning they have the same structure, have the same genes, although the copies may differ. P1-22 AUTOSOMES Pairs 1-22 called AUTOSOMES LSEX CHROMOSOMES Last pair (23 rd ) are SEX CHROMOSOMES 4 Male - XY

5. Recall that most cells are formed by mitosis. Cells divide resulting in two daughter cells that are genetically identical to the original cell. The chromosome number stays the same.

6. Cells can be divided into two major groups. 2. Gametes are your sex cells - sperm & egg Has half the number of chromosomes- Haploid (n) Human gamete has 23 chromosomes

7. Meiosis Meiosis is a form of nuclear division that divides a cell to form gametes for sexual reproduction and is important for two reasons: A) Meiosis reduces the number of chromosomes in a cell by ½. B) Meiosis creates opportunities for genes to be exchanged (cross over).

8. Meiosis is sometimes called “Reduction Division” because it reduces the normal chromosome number in half. Meiosis DIPLOID CELLHAPLOID CELL-----------------> 46 Chromosomes23 Chromosomes

9. This is unlike in Mitosis where the chromosome number stays the same. Mitosis = Asexual Reproduction DIPLOID CELL -----------------> 46 Chromosomes

10. The Meiosis Process Meiosis begins in Interphase where the cell is going through G1, S, and G2. INTERPHASE

11. The Meiosis Process When directed by certain chemicals the cell begins meiosis, instead of mitosis. Meiosis is divided into two stages: A) Meiosis I B) Meiosis II

13. Meiosis I – Prophase I The nuclear envelope of the cell begins to break up and starts to disappear. Chromosomes become visible.

14. Meiosis I – Prophase I Synapsis- Homologous chromosomes pair up (similar chromosomes from mom & dad). The formation is called a tetrad. Chromosome from Mom Chromosome from Dad

15. Meiosis I – Prophase I When tetrads get too close segments of genes can be exchanged (cross over.) This allows for more combinations of DNA to occur and even more diversity. Chromosome from Mom Chromosome from Dad

17. Meiosis I – Metaphase I The tetrads (homologous chromosomes) move to the center (equator) of the cell.

18. Meiosis I – Anaphase I Tetrads (homologous chromosomes) get pulled apart and moved to opposite sides.

19. Meiosis I – Telophase I The chromosomes gather at opposites sides and the cytoplasm divides to make two new cells.

20. Meiosis I – Cytokinesis I Two cells are made. Chromosomes stay wound up.

21. So far… The sister chromatids have NOT been pulled apart at their centromeres. Interphase does NOT occur again. The chromosomes are NOT copied.

22. Meiosis II – Prophase II Chromosomes are still visible (wound up). Spindle fibers surround the chromosomes. This occurs in both of the cells made during Meiosis I.

23. Meiosis II – Metaphase II The sister chromatids move to the center (equator) of the cell. This occurs in both of the cells made during Meiosis I.

24. Meiosis II – Anaphase II Sister chromatids get pulled apart at their centromeres and move to opposite sides. This occurs in both of the cells made during Meiosis I.

25. Meiosis II – Telophase II The chromosomes gather at opposites sides and the cytoplasm divides. This occurs in both of the cells made during Meiosis I.

26. Meiosis II – Cytokinesis II Two more cells are made (4 total). This occurs in both of the cells made during Meiosis I.

29. Meiosis Animation http ://biology.clc.uc.edu/graphics/bio104/meiomove.gif

30. Meiosis Summary We start with 1 diploid cell and make 4 haploid cells. A cell goes through 2 separate cell division processes (Meiosis I and II).

32. Meiosis Summary Parent cells begin with 46 chromosomes. In S phase the 46 chromosomes make identical copies to become sister chromatids (92 chromosomes).

33. 46 chromosomes 92 chromosomes

34. Meiosis Summary During Meiosis I homologous chromosomes are separated into 2 cells that now have 46 chromosomes. 92 chromosomes 46 chromosomes

35. Meiosis Summary Both cells from Meiosis I, go to Meiosis II. Now sister chromatids are separated into cells that each have 23 chromosomes. 46 chromosomes 23

36. Meiosis Summary A grand total of 4 cells each containing 23 chromosomes are formed!

38. Diversity in Meiosis When tetrads are formed during Meiosis I they can cross over segments of DNA. http://www.tokyo-med.ac.jp/genet/anm/mimov.gif

39. More Diversity in Meiosis The tetrads (homologous chromosomes) randomly line up at the equator and can separate independently. This is called independent assortment. A zygote is made by a male sperm joining with a female egg.

41. Even More Diversity in Meiosis Sexual reproduction involves the fusion of two gametes, resulting in offspring that are a genetic mixture of both parents. The actual fusion of an egg & sperm cell is called fertilization. Copy this into notes

42. Diversity! Combined all three processes: 1) crossing over, 2) independent assortment, 3) random fertilization, …create an almost infinite number of possible outcomes.

43. The process of making gametes is called gametogenesis. The process of Meiosis in males is known as spermatogenesis. It occurs in the testes. It creates 4 functional sperm.

45. Meiosis in Females The process of Meiosis in females is known as oogenesis. It occurs in the ovaries. It creates 1 functional egg and 3 polar bodies.

47. Mitosis vs. Meiosis

48. Involved in asexual reproduction. Occurs in body cells. Cells divide 1 time and make 2 cells. Involved in sexual reproduction. Occurs in sex cells. Cells divide 2 times and make 4 cells.

49. Mitosis vs. Meiosis No crossing over. Cells are diploid (46) All new cells made are identical. Yes crossing over. Cells are haploid (23) Not all new cells made are identical (DIVERSITY!).