Sexual Cell Reproduction Chapter 17

Transcription

1 Sexual Cell Reproduction Chapter 17 1

2 The Importance of Meiosis Meiosis is a two stage cell division in which the chromosome number of the parental cell is reduced by half. Meiosis is the process by which sex cells are formed. Sex cells are often called gametes (haploid chromosome number) Ex: human cell that has 46 chromosomes (Diploid), that undergoes meiosis and produce sex cells (gametes) that have 23 chromosomes (haploid). The union of 23 sperm chromosomes and 23 chromosomes from an egg cell will produce a 46 chromosome fertilized egg or zygote. 2

3 Organisms that reproduce sexually, show a greater range of characteristics within a species. Sexual ensures recombination of genes. (father s eyes, mother s hair) Paired chromosomes 23 from each parent are called homologous chromosomes. (similar in shape, size and gene arrangement) 3

4 Homologous Chromosomes 4

5 Stages of Meiosis Meiosis involves two cell divisions that produce four haploid cells. First division (meiosis I) the homologous chromosomes move to opposite poles. During this division diploid cells separate into two haploid cell. The second division (meiosis II) is marked by a separation of the two chromatids. 5

6 MEIOSIS Production of Gametes 6

7 Meiosis Facts 7

8 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) 8

9 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 9

10 Meiosis Occurs only in gonads (testes or ovaries) Male: Spermatogenesis Female: Oogenesis 10

11 Spermatogenesis Human sex cell n=23 n=23 n=23 Sperm 2n=46 Diploid (2n) n=23 n=23 Haploid (n) n=23 Meiosis I Meiosis II 11

12 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. 12

13 Interphase I Nucleus and nucleolus visible. Chromatin Nuclear membrane Cell membrane Nucleolus 13

14 Meiosis I 14

15 Stages of 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 15

16 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) 16

17 Prophase I - Synapsis Homologous chromosomes Sister chromatids Tetrad Sister chromatids 17

18 Homologous Chromosomes Pair of chromosomes (maternal and 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 18

19 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 19

20 Homologous Chromosomes eye color locus eye color locus hair color locus hair color locus Paternal Maternal 20

21 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 21

22 Crossing Over - Variation nonsister chromatids Tetrad chiasmata: site of crossing over variation 22

23 Sex Chromosomes XX chromosome - female XY chromosome - male 23

24 Prophase I Spindle fiber Centrioles Aster fibers 24

25 Prophase I Review Prophase I nuclear membrane dissolves, centrioles move to opposite poles, spindle fibers established. Chromosomes come together in pairs called tetrads. Synapsis the pairing of homologous chromosomes. Crossing over the exchange of genetic material between to homologous chromosomes. Often as tetrads come closer together the chromatids intertwine. The pairing of these homologous chromosomes is called synapsis and they sometimes exchange genetic material. 25

26 Crossing Over 26

27 Metaphase I Shortest phase Tetrads align on the metaphase plate Independent assortment occurs Chromosomes separate randomly to the poles of the cells 27

28 Metaphase I Independent assortment causes variation in the forming cells Orientation of homologous pair to poles is random Formula for determining variation: Formula: 2 n Example: : 2n = 4 then n = 2 thus 2 2 = 4 combinations 28

29 Metaphase I OR Metaphase plate Metaphase plate 29

30 Question: In terms of Independent Assortment, how many different combinations of sperm could a human male produce? 30

31 Answer Formula: 2 n Human chromosomes: 2n = 46 n = = ~8 million combinations 31

32 Metaphase I Review Metaphase I homologous chromosomes attach themselves to spindle fibers and line up along the equatorial plate. 32

33 Anaphase I Homologous chromosomes separate and move towards the poles Sister chromatids remain attached at their centromeres 33

34 Anaphase I 34

35 Anaphase I Review Anaphase I homologous chromosomes move toward opposite poles. This process is known as segregation. At this point one member of each homologous pair will be found in each of the new cells. The diploid mother cell becomes two haploid daughter cells. Each chromosome remains double stranded. 35

36 Telophase I Each pole now has haploid set of chromosomes 1n = 23 (human) Cytokinesis occurs and two haploid daughter cells are formed 36

37 Telophase I 37

38 Telphase I Review Telophase I the cytoplasm divides forming two cells. Nuclear membrane begins to form around the chromosomes within each of the two cells. 38

39 Meiosis I 39

40 40

41 Meiosis II 41

42 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 42

43 Prophase II Same as prophase in mitosis Nuclear envelope breaks down and the spindle apparatus forms 43

44 Metaphase II Same as metaphase in mitosis Chromatids lined up at equator Metaphase plate Metaphase plate 44

45 Anaphase II Same as anaphase in mitosis Sister chromatids separate 45

46 Telophase II Same as telophase in mitosis Nuclei reform Cytokinesis occurs Remember: Four haploid daughter cells produced Gametes = sperm or egg 46

47 Telophase II 47

48 Meiosis n=2 sex cell n=2 n=2 Sperm 2n=4 diploid (2n) n=2 n=2 Haploid (n) n=2 Meiosis I Meiosis II 48

49 49

50 Meiosis II Review Occurs at approx. same time in each of the haploid daughter cells. Pairs of chromatids separate and move to opposite poles. Prophase II nuclear membrane disolve and spindle fibers begin to form. Metaphase II signalled by the arrangement of the chromosomes, each with two chromatids, along equatorial plate. 50

51 Anaphase II movement of sister chromatids to opposite poles. Nuclear membrane begind to form around the chromosomes. Telophase II second division of cytoplasm, four daughter cells are produced. 51

52 Meiosis II 52

53 Genetic Variation 53

54 Variation Important to the survival of populations Aids in natural selection Strongest individuals are able to survive and reproduce 54

55 Question What are the three sources of genetic variation in sexual reproduction? 55

56 Answer: 1. Crossing Over (Prophase I) 2. Independent Assortment (Metaphase I) 3. Random Fertilization (sperm joins with egg) 56

57 Remember: Variation is helpful to the survival of a species! 57

58 Question: A diploid cell containing 20 chromosomes (2n = 20) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? 58

59 Answer: 10 chromosomes (haploid) 1n = 10 59

60 Karyotype A method of organizing the chromosomes of a cell in relation to number, size, and type. 60

61 61

62 Fertilization The fusion of a sperm and egg to form a zygote. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote 62

63 Question: A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? 63

64 Answer: Four cells with 10 chromosomes each 64

65 Comparing Mitosis and Meiosis Mitosis Asexual reproduction. Results in two daughter cells that are identical to each other. Same genetic info and number of chromosomes as parent. 65

66 Meiosis Sexual reproduction. Results in four daughter cells that are different from each other and from the parent cell. Carry only half the number of chromosomes from the parent. 66

67 Comparing Mitosis and Meiosis 67

68 Comparing Mitosis and Meiosis 68

69 Comparing Mitosis and Meiosis 69

70 Comparing Mitosis and Meiosis 70

71 Development of Male and Female Gametes Gametogenesis the formation of gametes (sex cells) in animals. Cytoplasm of female gametes do not divide equally after each nuclear division. As shown in figure 9, one of the daughter cells called the ootid receives most of the cytoplasm. The other cells the polar bodies die. 71

72 Only one ovum (egg) is produced from meiosis. In contrast with sperm there is an equal division of the cytoplasm. 72

73 Development of Male and Female Gametes 73

74 Sex chromosomes are pairs of chromosomes that determine the sex of an individual. Autosomes are chromosomes not involved with sex determination. 74

75 Sex Chromosomes 75

76 Summary 76

77 Homework Review Questions Page 453 Questions

78 Activity 78

79 Abnormal Meiosis Nondisjunction- failure of a pair of homologous chromosomes to separate properly during meiosis. Polyploidy a condition in which an organism has more than two complete sets of chromosomes. Trisomy there are three homologous chromosomes in place of a pair of homologous. Monosomy there is a single chromosome in place of a homologous pair. 79

80 Nondisjunction Disorders Down Syndrome a trisomic condition, three copies of chromosome 21. Older women having babies are at higher risk. Traits include: round full face, enlarged and creased tongue, short height, large forehead. Associated with mental retardation 80

81 Down Syndrome 81

82 Karyotype of Normal Male vs. Down Syndrome Female 82

83 Nondisjunction Disorders 83

84 Turner Syndrome Monosmis disorder produces a female with a single X chromosome. In the egg both X chromosomes move to the same pole. Individuals appear female, but do not developed sexually and tend to be short and have thick necks. 84

85 Klinefelter Syndrome Child inherits two X chromosomes and asingle Y. Child appears to be male at birth, however as he enters maturity, he begins producing high levels of female hormones. Males are sterile. 85

86 Detects abnormal meiosis. Karotype Charts Mix a small sample of tissue with a solution that simulates mitotic division. A different solution is added to stop the division at metaphase. Since chromosomes are in their most condensed form, their size, length, and centromere location is best to see. The metaphase cells are placed on a slide and then stained. A photograph is taken, the image enlarged and each chromosome is cut out and paired up with its homologue. They are all aligned in decreasing order. Sex chromosomes always placed last. 86

87 Activity 87

88 Summary 88

89 Homework Review Questions Page 462 Questions

90 Lab 90

91 91