Exploring Meiosis: Facts and Stages in Gamete Production

1. Production of Gametes MEIOSIS

2. Meiosis Facts

3. Meiosis • The form of cell division by which gametes, with half the number of chromosomes, are produced • Diploid (2n)  haploid (n) • Meiosis is sexual reproduction • Two divisions (meiosis I and meiosis II)

4. Meiosis • Sex cells divide to produce gametes (sperm or egg) • Gametes have half the number of chromosomes • Meiosis is similar to mitosis with some chromosomal differences

5. Meiosis • Occurs only in gonads (testes or ovaries) • Male: Spermatogenesis • Female: Oogenesis

6. n=23 Human sex cell Sperm 2N=46 n=23 4N=92 Haploid (n) n=23 2N=46 After S phase Of cell cycle n=23 Meiosis I Meiosis II Spermatogenesis

7. Interphase I • Similar to mitosis interphase • Chromosomes replicate(S phase) • Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres • Centriole pairs also replicate.

8. Interphase I • Nucleus and nucleolus visible. Chromatin Nuclear membrane Cell membrane Nucleolus

9. Meiosis I

10. Stages of Meiosis I • Cell division thatreduces the chromosome number by one-half • Four phases: a. Prophase I b. Metaphase I c. Anaphase I d. Telophase I

11. Prophase I • Longest and most complex phase (90%) • Chromosomes condense • Synapsis occurs: homologous chromosomes come together to form a tetrad • Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids)

12. Homologous chromosomes Sister chromatids Sister chromatids Tetrad Prophase I - Synapsis

13. Homologous Chromosomes • Pair of chromosomes (maternal or paternal) that are similar in shape and size • Homologous pairs (tetrads)carry genes controlling the same inherited traits • Each locus(position of a gene)is in the same position on homologues

14. Homologous Chromosomes • Humans have 23 pairs of homologous chromosomes • First 22 pairs of chromosomes called autosomes • Last pair called sex chromosomes • XX female or XY male

15. eye color locus eye color locus hair color locus hair color locus Paternal Maternal Homologous Chromosomes

16. Crossing Over • Crossing over (variation) may occur between nonsister chromatids at the chiasmata • Crossing over: segments of nonsister chromatids break and reattach to the other chromatid • Chiasmata (chiasma) are the sites of crossing over

17. Tetrad nonsister chromatids chiasmata: site of crossing over Crossing Over - Variation variation

18. Sex Chromosomes XX chromosome - female XY chromosome - male

19. Spindle fiber Centrioles Aster fibers Prophase I

20. Metaphase I • Shortest phase • Tetrads align on the metaphase plate • Independent assortment occurs • Chromosomes separate randomly to the poles of the cells

21. Metaphase I • Independent assortment causes variation in the forming cells • Orientation of homologous pair to poles is random • Formula for determining variation: Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations

22. OR Metaphase plate Metaphase plate Metaphase I

23. Question: • In terms of Independent Assortment, how many different combinations of sperm could a human male produce?

24. Answer • Formula: 2n • Human chromosomes: 2n = 46 n = 23 223 = ~8 million combinations

25. Anaphase I • Homologous chromosomes separate and move towards the poles • Sister chromatids remain attached at their centromeres

26. Anaphase I

27. Telophase I • Each pole now has haploid set of chromosomes • 1n = 23 (human) • Cytokinesis occurs and two haploid daughter cells are formed

28. Telophase I

30. Meiosis II

31. Meiosis II • Short or No interphase II • DNA NOT replicated again • Remember: Meiosis II is similar to mitosis • Prophase I, Metaphase II, Anaphase II, and Telophase II

32. Prophase II • Same as prophase in mitosis • Nuclear envelope breaks down and the spindle apparatus forms

33. Metaphase plate Metaphase plate Metaphase II • Same as metaphase in mitosis • Chromatids lined up at equator

34. Anaphase II • Same as anaphase in mitosis • Sister chromatids separate

35. Telophase II • Same as telophase in mitosis • Nuclei reform • Cytokinesis occurs • Remember: Four haploid daughter cells produced • Gametes = sperm or egg

36. Telophase II

37. n=2 sex cell Sperm 2N=4 n=2 4N=8 Haploid (n) n=2 2N=4 n=2 Meiosis I Meiosis II Meiosis

39. Genetic Variation

40. Variation • Important to the survival of populations • Aids in natural selection • Strongest individuals are able to survive and reproduce

41. Question • What are the three sources of genetic variation in sexual reproduction?

42. Answer: • Crossing Over (Prophase I) 2. Independent Assortment (Metaphase I) 3. Random Fertilization(sperm joins with egg)

43. Remember:Variation is helpful to the survival of a species!

44. Question: • A diploid cell containing 20 chromosomes(2n = 20)at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

45. Answer: • 10 chromosomes (haploid) • 1n = 10

46. Karyotype • A method of organizing the chromosomes of a cell in relation to number, size, and type.

48. sperm n=23 n=23 egg 2n=46 zygote Fertilization • The fusion of a sperm and egg to form a zygote. • A zygote is a fertilized egg

49. Question: • A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

50. Answer: • Four cells with 10 chromosomes each