Meiosis and Sexual Reproduction. Chapter 10

Transcription

1 Meiosis and Sexual Reproduction Chapter 10

2 Impacts, Issues: Why Sex? An adaptive trait tends to spread more quickly through a sexually reproducing population than through an asexually reproducing one

3 Fig. 10-1b, p. 154

4 10.1 Introducing Alleles Asexual reproduction produces genetically identical copies of a parent (clones) Sexual reproduction introduces variation in the combinations of traits among offspring

5 Genes and Alleles Genes are regions in an organism s DNA that encode information about heritable traits In sexual reproduction, pairs of genes are inherited on pairs of chromosomes, one maternal and one paternal Alleles are different forms of the same gene Offspring of sexual reproducers inherit new combinations of alleles, the basis of traits

6 Chromosome Pair: Maternal and Paternal

7 10.1 Key Concepts Sexual vs Asexual Reproduction In asexual reproduction, one parent transmits its genetic information to offspring In sexual reproduction, offspring typically inherit information from two parents who differ in their alleles Alleles are different forms of the same gene; they specify different versions of a trait

8 10.2 What Meiosis Does Meiosis A nuclear division mechanism that precedes cytoplasmic division of immature reproductive cells in sexually-reproducing eukaryotic species

9 Halving the Diploid Number A diploid cell has two nonidentical copies of every chromosome (except XY sex chromosomes) Humans have 23 pairs of homologous chromosomes Meiosis in germ cells halves the diploid number of chromosomes (2n) to the haploid number (n), producing haploid gametes Eggs and sperm have 23 unpaired chromosomes

10 Gamete Production Gametes are produced in specialized reproductive structures or organs

11 Fig. 10-3a, p. 156

12 anther (where sexual spores that give rise to sperm form) ovules inside an ovary (where sexual spores that give rise to eggs form) a Flowering plant Fig. 10-3a, p. 156

13 Fig. 10-3b, p. 156

14 testis (where sperm originate) b Human male Fig. 10-3b, p. 156

15 Fig. 10-3c, p. 156

16 ovary (where eggs develop) c Human female Fig. 10-3c, p. 156

17 Restoring the Diploid Number Human gametes (eggs and sperm) have 23 chromosomes one of each homologous pair The diploid number (23 pairs) is restored at fertilization, when two haploid gametes fuse and form a diploid zygote, the first cell of a new individual

18 Human Chromosomes: Homologous Pairs

19 Two Divisions, Not One In meiosis, DNA is replicated once and divided twice (meiosis I and meiosis II), forming four haploid nuclei In meiosis I, each duplicated homologous chromosome is separated from its partner In meiosis II, sister chromatids are separated

20 Two Divisions, Not One Meiosis I Meiosis II

21 each chromosome in the cell pairs with its homologous partner then the partners separate p. 157

22 one chromosome (duplicated) two chromosomes (unduplicated) p. 157

23 10.3 Visual Tour of Meiosis

24 10.3 Visual Tour of Meiosis

25 Meiosis I plasma membrane newly forming microtubules of the spindle one pair of homologous chromosomes breakup of nuclear envelope centrosome with a pair of centrioles, moving to opposite sides of nucleus A Prophase I B Metaphase I C Anaphase I D Telophase I Fig. 10-5a, p. 158

26 Fig. 10-5a (1), p. 158

27 Fig. 10-5a (1), p. 158

28 Fig. 10-5a (2), p. 158

29 Fig. 10-5a (2), p. 158

30 Fig. 10-5a (3), p. 158

31 Fig. 10-5a (3), p. 158

32 Fig. 10-5a (4), p. 158

33 Fig. 10-5a (4), p. 158

34 Fig. 10-5b (1), p. 159

35 Fig. 10-5b (2), p. 159

36 Fig. 10-5b (3), p. 159

37 Fig. 10-5b (4), p. 159

38 Meiosis I plasma membrane newly forming microtubules of the spindle one pair of homologous chromosomes breakup of nuclear envelope centrosome with a pair of centrioles, moving to opposite sides of nucleus A Prophase I B Metaphase I C Anaphase I D Telophase I Stepped Art Fig. 10-5a, p. 158

39 Meiosis II There is no DNA replication between the two nuclear divisions. E Prophase II F Metaphase II G Anaphase II H Telophase II Stepped Art Fig. 10-5b, p. 159

40 Animation: Meiosis step-by-step

41 Key Concepts Stages of Meiosis Meiosis reduces the chromosome number Meiosis occurs only in cells set aside for sexual reproduction Meiosis sorts a reproductive cell s chromosomes into four haploid nuclei, which are distributed to descendent cells by cytoplasmic division

42 10.4 How Meiosis Introduces Variation in Traits Crossovers and the random sorting of chromosomes in meiosis introduce novel combinations of alleles into gametes, resulting in new combinations of traits among offspring

43 Crossing Over in Prophase I Crossing over The process by which a chromosome and its homologous partner exchange heritable information in corresponding segments Occurs during condensation in prophase I

44 Crossing Over Between Homologous Chromosomes

45 Fig. 10-6a, p. 160

46 Fig. 10-6b, p. 160

47 A A a a B Here, we focus on only two genes. One gene has alleles A and a; the other has alleles B and b. B B b b Fig. 10-6b, p. 160

48 Fig. 10-6c, p. 160

49 crossover C Close contact between the homologous chromosomes promotes crossing over between nonsister chromatids, so paternal and maternal chromatids exchange segments. Fig. 10-6c, p. 160

50 Fig. 10-6d, p. 160

51 D Crossing over mixes up paternal and maternal alleles on homologous chromosomes. A A a a B B b b Fig. 10-6d, p. 160

52 Animation: Crossing over

53 Segregation of Chromosomes into Gametes Homologous chromosomes can be attached to either spindle pole in prophase I, so each homologue can be packaged into either one of the two new nuclei Random assortment produces (8,388,608) possible combinations of homologous chromosomes

54 Random Assortment

55 A Alignment in nucleus at metaphase I B Alignments in two nuclei at metaphase II C Nuclei of the four resulting gametes Possible lineup #1 Possible lineup #2 Possible lineup #3 Possible lineup #4 Fig. 10-7, p. 161

56 A Alignment in nucleus at metaphase I B Alignments in two nuclei at metaphase II C Nuclei of the four resulting gametes Possible lineup #1 Possible lineup #2 Possible lineup #3 Possible lineup #4 Stepped Art Fig. 10-7, p. 161

57 Animation: Random alignment

58 10.4 Key Concepts: Chromosome Recombinations and Shufflings During meiosis, each pair of maternal and paternal chromosomes swaps segments Then, each chromosome is randomly segregated into one of the new nuclei Both processes lead to novel combinations of alleles and traits among offspring

59 10.5 From Gametes to Offspring Aside from meiosis, the details of gamete formation and fertilization differ among plants and animals

60 Gamete Formation in Plants Sporophytes Diploid bodies with specialized structures that form spores (haploid cells) that give rise to gametophytes through mitosis Gametophytes A multicelled haploid body inside which one or more gametes form

61 Life Cycles of Plants and Animals

62 Fig. 10-8a, p. 162

63 mitosis zygote (2n) multicelled sporophyte (2n) fertilization DIPLOID HAPLOID meiosis gametes (n) mitosis multicelled gametophyte (n) mitosis spores (n) a Plant life cycle Fig. 10-8a, p. 162

64 Fig. 10-8b, p. 162

65 mitosis zygote (2n) multicelled body (2n) fertilization DIPLOID HAPLOID meiosis b Animal life cycle gametes (n) Fig. 10-8b, p. 162

66 Gamete Formation in Animals Males Meiosis of primary spermatocytes produces four haploid spermatids, which mature into sperm Females Meiosis of a primary oocyte forms cells of different sizes; the secondary oocyte gets most of the cytoplasm and matures into an ovum (egg); other cells (polar bodies) get little cytoplasm and degenerate

67 Sperm Formation in Animals

68 sperm (mature, haploid male gametes) secondary spermatocytes (haploid) diploid male germ cell A Growth primary spermatocyte (diploid) B Meiosis I and cytoplasmic division spermatids (haploid) C Meiosis II and cytoplasmic division Fig. 10-9, p. 163

69 sperm (mature, haploid male gametes) secondary spermatocytes (haploid) diploid male germ cell A Growth primary spermatocyte (diploid) B Meiosis I and cytoplasmic division spermatids (haploid) C Meiosis II and cytoplasmic division Stepped Art Fig. 10-9, p. 163

70 Animation: Sperm formation

71 Egg Formation in Animals

72 Fig (left), p. 163

73 first polar body (haploid) three polar bodies (haploid) oogonium (diploid female germ cell) primary oocyte (diploid) secondary oocyte (haploid) ovum (haploid) A Growth B Meiosis I and cytoplasmic division C Meiosis II and cytoplasmic division Fig (left), p. 163

74 first polar body (haploid) three polar bodies (haploid) oogonium (diploid female germ cell) primary oocyte (diploid) secondary oocyte (haploid) ovum (haploid) A Growth B Meiosis I and cytoplasmic division C Meiosis II and cytoplasmic division Stepped Art Fig , p. 163

75 Animation: Egg formation

76 More Shufflings at Fertilization Chance combinations of maternal and paternal chromosomes through fertilization produce a unique combination of genetic information Fertilization The fusion of two haploid gametes (sperm and egg) resulting in a diploid zygote

77 10.5 Key Concepts: Sexual Reproduction in Context of Life Cycles Gametes form by different mechanisms in males and females In most plants, spore formation and other events intervene between meiosis and gamete formation

78 10.6 Mitosis and Meiosis An Ancestral Connection? Though they have different results, mitosis and meiosis are fundamentally similar processes Meiosis may have evolved by the remodeling of existing mechanisms of mitosis

79 Comparing Mitosis and Meiosis

80 Fig a (1), p. 165

81 Fig a (2), p. 165

82 Fig a (3), p. 165

83 Fig b, p. 165

84 one diploid nucleus two diploid nuclei Prophase Metaphase Anaphase Telophase Chromosomes condense. Nuclear envelope breaks up. Bipolar spindle forms; it attaches chromosomes to spindle poles. Chromosomes align midway between spindle poles. Sister chromatids separate as they are pulled toward spindle poles. Chromosome clusters arrive at spindle poles. Chromosomes decondense. New nuclear envelopes form. Fig b, p. 165

85 Fig c, p. 164

86 one diploid nucleus two haploid nuclei Prophase I Chromosomes condense. Homologous chromosomes pair. Bipolar spindle forms; it attaches chromosomes to spindle poles. Crossovers occur. Nuclear envelope breaks up. Metaphase I Anaphase I Telophase I Chromosomes align midway between spindle poles. Homologous chromosomes separate as they are pulled toward spindle poles. Chromosome clusters arrive at spindle poles. New nuclear envelopes form. Chromosomes decondense. Fig c, p. 164

87 Fig d, p. 165

88 two haploid nuclei four haploid nuclei Prophase II Metaphase II Anaphase II Telophase II Chromosomes condense. Bipolar spindle forms; it attaches chromosomes to spindle poles. Nuclear envelope breaks up. Chromosomes align midway between spindle poles. Sister chromatids separate as they are pulled toward spindle poles. Chromosome clusters arrive at spindle poles. New nuclear envelopes form. Chromosomes decondense. Fig d, p. 165

89 10.6 Key Concepts Mitosis and Meiosis Compared Meiosis may have originated by evolutionary remodeling of mechanisms that already existed for mitosis, and before that, for repairing damaged DNA

90 Animation: Comparing mitosis and meiosis

91 Animation: Generalized life cycles

92 Animation: Meiosis

93 Animation: Meiosis I and II

94 Animation: Reproductive organs

95 Video: Why sex?