1. What is Meiosis Exactly? Meiosis is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells such as gametes or spores Meiosis is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells such as gametes or spores There are 2 stages of meiosis, Meiosis 1 and meiosis 2 There are 2 stages of meiosis, Meiosis 1 and meiosis 2 Meiosis 1 and 2 create 4 haploid cells all together Meiosis 1 and 2 create 4 haploid cells all together

2. IV. Meiosis – Reduction Division

3. Stages of Meiosis 1

4. Stages of Meiosis 2

5. Interphase before replication

6. Interphase after replication

7. I. Meiosis (Reduction Division) A. Meiosis I 1. Prophase I a. chromosomes become distinct a. chromosomes become distinct b. nucleolus and nuclear membrane b. nucleolus and nuclear membrane disappear and spindle fibers appear disappear and spindle fibers appear

8. Prophase I c. spindle fibers appear d.synapsis–homologous chromosomes Line up together

9. Prophase I e. crossing over may occur 1) portions of chromatid from one 1) portions of chromatid from one parent break off and attach to a parent break off and attach to a homologous chromatid from the homologous chromatid from the other parent other parent 2) results in genetic recombination 2) results in genetic recombination

10. 2. Metaphase I a. chromosomes line up along the midline a. chromosomes line up along the midline b. sister chromatids do not separate b. sister chromatids do not separate

11. 3. Anaphase I a. at random, one member of each a. at random, one member of each homologous pair moves to the homologous pair moves to the opposite poles (independent opposite poles (independent assortment assortment

12. 4. Telophase I and Cytokinesis I a. chromosomes reach opposite poles a. chromosomes reach opposite poles b. cytokinesis begins b. cytokinesis begins

13. Telophase I c. resulting cells have the n or haploid number of chromosomes 1) one member of each homologous pair with two attached chromatids 1) one member of each homologous pair with two attached chromatids d. each new cell contains ½ the d. each new cell contains ½ the the number of chromosomes as the original cell the number of chromosomes as the original cell

14. B. Meiosis II 1. Prophase II a. spindle form and chromosomes a. spindle form and chromosomes begin to move toward the mid- line of begin to move toward the mid- line of the cell the cell

15. . Metaphase II a. chromosomes move to the mid- line of the dividing cell a. chromosomes move to the mid- line of the dividing cell

16. 3. Anaphase II a. chromatids separate and move to the opposite poles of the cell a. chromatids separate and move to the opposite poles of the cell

17. 4. Telophase II a. nuclear membrane forms around the nucleus in each cell a. nuclear membrane forms around the nucleus in each cell b. each resulting cell contains the b. each resulting cell contains the n number of chromosomes n number of chromosomes

19. Meiosis 1 and Meiosis 2

20. Animation of meiosis

21. C. Chart Comparing Mitosis and Meiosis Mitosis 2 cells result 2 cells result One division One division 2n number of chromosomes in resulting cells 2n number of chromosomes in resulting cellsMeiosis 4 cells result 4 cells result Two divisions Two divisions n number of chromosomes in resulting cells n number of chromosomes in resulting cells

23. D. Genetic Recombination and Variation 1. independent assortment 2. crossing over 3. random fusion of sperm and egg

24. E. Formation of Gametes 1. spermatogenesis – results in 4 viable sperm viable sperm 2. oogenesis – results in 1 egg and 3 polar bodies 3 polar bodies

25. Spermatogenesis

26. Oogenesis

28. Fertilization [sperm (n) + egg (n)  zytote (2n) ]

29. F. Sexual vs Asexual Reproduction Asexual One parent One parent Identical genetic material Identical genetic material Mitosis, budding, binary fission Mitosis, budding, binary fissionSexual Two parents Two parents Different genetic material Different genetic material Meiosis + fertilization Meiosis + fertilization

30. Meiosis creates genetic variation During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n) During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n) Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossing over. Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossing over. No daughter cells formed during meiosis are genetically identical to either mother or father During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring.

31. Karyotype Karyotype 1. picture of homologous chromo- 1. picture of homologous chromo- somes and sex chromosomes somes and sex chromosomes 2. used to detect chromosome 2. used to detect chromosome abnormalities abnormalities

32. Homologues Chromosomes exist in homologous pairs in diploid cells. Chromosomes exist in homologous pairs in diploid cells. Exception: Sex chromosomes (X, Y). Other chromosomes are known as autosomes, they have homologues.

33. Human Chromosomes

35. In humans … 23 chromosomes donated by each parent (total = 46 or 23 pairs). 23 chromosomes donated by each parent (total = 46 or 23 pairs). Gametes (sperm/ova): Gametes (sperm/ova): Contain 22 autosomes and 1 sex chromosome. Contain 22 autosomes and 1 sex chromosome. Are haploid (haploid number “n” = 23 in humans). Are haploid (haploid number “n” = 23 in humans). Fertilization/syngamy results in zygote with 2 haploid sets of chromosomes - now diploid. Fertilization/syngamy results in zygote with 2 haploid sets of chromosomes - now diploid. Diploid cell; 2n = 46. (n=23 in humans) Diploid cell; 2n = 46. (n=23 in humans) Most cells in the body produced by mitosis. Most cells in the body produced by mitosis. Only gametes are produced by meiosis. Only gametes are produced by meiosis.

36. In humans e.g. 23 chromosomes in haploid 2n = 46; n = 23 2 n = 2 23 = ~ 8 million possible combinations!

37. Random fertilization At least 8 million combinations from Mom, and another 8 million from Dad … >64 trillion combinations for a diploid zygote!!!

38. Karyotyping- normal

40. some of the previous slides came from the following website some of the previous slides came from the following website http://cchs.churchill.k12.nv.us/marshk/Not es/meiosis.ppt#352,39,Slide 39 http://cchs.churchill.k12.nv.us/marshk/Not es/meiosis.ppt#352,39,Slide 39

41. www.biology.Arizona.edu/ www.biology.Arizona.edu/ www.biology.Arizona.edu