Meiosis is a type of cell division that produces gametes, such as sperm and egg cells, with half the number of chromosomes as regular body cells. This involves two rounds of division called Meiosis I and Meiosis II. In Meiosis I, homologous chromosomes pair up and may exchange genetic material through crossing over. This results in two daughter cells each with half the original number of chromosomes. Meiosis II then separates the sister chromatids, resulting in four haploid gametes total. This ensures fertilization restores the full chromosome number.

1. Meiosis

2. Pg. Title: Meiosis Date: 01/27/10 Obj: Describe how spermatogenesis & oogenesis occur through meiosis. Ess: Explain how sperm & egg contain only 23 chromosomes each through meiosis.

3. Pg. Title: Meiosis I & II Date: 1/27/10 In: Almost all human cells contain 46 chromosomes. If a sperm has 46 chromosomes & an egg has 46 chromosomes then how many chromosomes will the “baby” have? Is that a good thing???

4. M I T O S I S M E I O S I S I M E I O S I S II

5. Asexual vs. Sexual Reproduction Asexual Reproduction: Binary Fission & Mitosis Produces cells that are identical copies of parent cell Sexual Reproduction: Meiosis Combines genetic material from 2 parents (sperm & egg) so offspring are genetically different from parents

6. How many chromosomes in human cells? Humans have 46 (23 pairs) chromosomes 44 chromosomes (22pairs): autosomal 2 chromosomes (1 pair): sex chromosomes (XX, XY) 2 sets of chromosomes 1 from mom & 1 from dad

7. How many chromosomes in human cells? All human cells have 46 chromosomes except for the egg & sperm Egg = 23 Sperm = 23 All other cells = 46 (2 sets of chromosomes) (liver, cardiac, blood cells) Diploid (2N) = 2 sets of chromosomes (46) Haploid (1N) = 1 set of chromosomes (23) N = haploid # 1 set of chromosomes

8. If egg and sperm had same number of chromosomes as other body cells . . . baby would have too many chromosomes! No Good! The Problem!

9. The Solution! Much Better! If egg and sperm had half the number of chromosomes than other body cells . . . baby would have just the right amount!

10. Meiosis Makes 4 cells genetically different from parent cell & from each other Production of gametes = sperm & egg Contain half the number of chromosomes = haploid (1N) Called the “reduction” division = Diploid Haploid 2N 1N Used in sexual reproduction 46 23

11. Meiosis allows sex cells to join to form a complete set of chromosomes/instructions to make a new organism. Copyright Pearson Prentice Hall Haploid gametes MEIOSIS Diploid zygote MITOSIS Diploid adult Not in notes

12. You tell me! Drosophila (fruit flies) Diploid: 2N = 8 Haploid: 1N = ? Lettuce Diploid: 2N = 8 Haploid: 1N = ? Goldfish: Diploid: 2N = ? Haploid: 1N = 48 Not in notes

13. Meiosis is a special kind of cell division that halves the number of chromosomes in gametes. This insures that the correct number of chromosomes are passed on. Copyright Pearson Prentice Hall Not in notes

14. The goal is to make haploid sex cells Copyright Pearson Prentice Hall 1 st division 2 nd division Not in notes

15. Where does it happen? In males- in the testes In females- in the ovaries

16. Homologous Chromosomes Homologous chromosomes sister chromatids paternal sister chromatids maternal Tetrad

17. Check out the tetrads!

18. Homologous Chromosomes Pair of chromosomes - maternal & paternal - similar in shape & size. Homologous pairs – tetrads - carry genes controlling the same inherited traits. Each locus - position of a gene - is in the same position on tetrads.

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

20. Gametes: Sperm or egg Copyright Pearson Prentice Hall

21. Gametogenesis Spermatogenesis: production of sperm

22. Gametogenesis Oogenesis: production of egg Meiosis I Meiosis II

24. Remember, humans have 23 pairs of chromosomes……………. Pairs 1-22 are said to be Homologous chromosomes (they go together because they have same traits but are not identical) Copyright Pearson Prentice Hall gene for eye color gene for hair color

25. Phases of Meiosis Meiosis I Copyright Pearson Prentice Hall Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Interphase I Meiosis I

26. Phases of Meiosis Cells undergo a round of DNA replication, forming duplicate chromosomes. Copyright Pearson Prentice Hall Interphase I

27. Phases of Meiosis Each chromosome pairs with its corresponding homologous chromosome to form a tetrad . There are 4 chromatids in a tetrad . Copyright Pearson Prentice Hall MEIOSIS I Prophase I

28. Phases of Meiosis Crossing-over: Homologous chromosomes exchange portions of their chromatids in a process Crossing-over produces new combinations of alleles. Copyright Pearson Prentice Hall

29. Synapsis and Crossing Over in Action Copyright Pearson Prentice Hall

30. Phases of Meiosis Spindle fibers attach to the chromosomes. Tetrads line up in the middle: metaphase plate. Copyright Pearson Prentice Hall MEIOSIS I Metaphase I

31. Phases of Meiosis Spindle fibers pull the homologous chromosomes toward opposite ends of the cell. Tetrads split into single chromosomes. Copyright Pearson Prentice Hall MEIOSIS I Anaphase I

32. Phases of Meiosis Nuclear membranes form. Cell separates into two cells. The two cells produced have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I. Copyright Pearson Prentice Hall MEIOSIS I Telophase I and Cytokinesis

33. Phases of Meiosis Meiosis II The two cells produced by meiosis I now enter a second meiotic division. Unlike meiosis I, neither cell goes through chromosome replication. Each of the cell’s chromosomes has 2 chromatids. Copyright Pearson Prentice Hall

34. Phases of Meiosis Meiosis II Copyright Pearson Prentice Hall Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Telophase I and Cytokinesis I Meiosis II

35. Phases of Meiosis Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell. Copyright Pearson Prentice Hall MEIOSIS II Prophase II

36. Phases of Meiosis The chromosomes line up in the center of cell. Copyright Pearson Prentice Hall MEIOSIS II Metaphase II

37. Phases of Meiosis The sister chromatids separate and move toward opposite ends of the cell. Copyright Pearson Prentice Hall MEIOSIS II Anaphase II

38. Phases of Meiosis Meiosis II results in four haploid (N) daughter cells. Copyright Pearson Prentice Hall MEIOSIS II Telophase II and Cytokinesis

39. Gametes: Sperm or egg Copyright Pearson Prentice Hall

40. Gametogenesis Spermatogenesis: production of sperm

41. Gametogenesis Oogenesis: production of egg Meiosis I Meiosis II

43. Gamete Formation Gamete Formation In male animals, meiosis results in four equal-sized gametes called sperm. Copyright Pearson Prentice Hall

44. Gamete Formation In many female animals, only one egg results from meiosis. The other three cells, called polar bodies , are usually not involved in reproduction. Copyright Pearson Prentice Hall

45. Interphase I DNA replicates forming duplicate homologous chromosomes. (S phase). Centriole pairs also replicate. Nucleus & nucleolus visible.

46. Meiosis I Cell division that reduces the chromosome number by one-half. four phases : a. prophase I b. metaphase I c. anaphase I d. telophase I & cytokinesis Prophase I Metaphase I Anaphase I Telophase I & Cytokinesis

47. Prophase I Longest and most complex phase (90%). Homologous chromosomes come together to form a tetrad = Synapsis Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids) .

48. Prophase I - Synapsis Homologous chromosomes sister chromatids paternal sister chromatids maternal Tetrad

49. Prophase I centrioles spindle fiber spindle fibers

50. Crossing Over Segments of non-sister chromatids break and reattach to the other chromatid . May occur between non-sister chromatids at the chiasmata . Chiasmata (chiasma) - sites of crossing over .

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

52. Metaphase I Shortest phase Tetrads align on the metaphase plate . INDEPENDENT ASSORTMENT OCCURS: Orientation of homologous pair to poles is random. Leads to variation

53. Metaphase I metaphase plate OR metaphase plate

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

55. Anaphase I

56. Telophase I Each pole now has haploid (1n) set of chromosomes . Cytokinesis occurs and two haploid daughter cells are formed.

57. Telophase I

58. Meiosis II No interphase II four phases : a. prophase II b. metaphase II c. anaphase II d. telophase II & cytokinesis Similar to mitosis

59. Prophase II Centrioles separate Nuclear envelope disappears

60. Prophase II

61. Metaphase II Sister chromatids line up at metaphase plate Spindle fibers attach at centromere

62. Metaphase II metaphase plate metaphase plate

63. Anaphase II sister chromatids separate & move toward opposite ends of the cell

64. Anaphase II

65. Telophase II Nuclei form. Cytokinesis occurs. Four haploid daughter cells produced. gametes = sperm or egg

66. Telophase II

68. Mitosis vs. Meiosis Mitosis Meiosis Diploid (2n) Parent Cells Replication of Chromosomes – double the # End of Mitosis & Meiosis I Results in 2 diploid (2n) daughter cells End of Meiosis II Results in 4 haploid (n) daughter cells

69. Pg. 143 Meiosis Vocab Word Description Picture Homologous chromosomes Tetrads Crossing over Haploid Diploid Locus Spermatogenesis Oogenesis