1. Meiosis Honors Biology

2. Introduction to Heredity Offspring acquire genes from parents by inheriting chromosomes Inheritance is possible because: –Sperm and ova carrying each parent’s genes are combined in the nucleus of the fertilized egg

3. Actual transmission of genes depends on the behavior of chromosomes Chromosomes-organizational unit of hereditary material in the nucleus of eukaryotic organisms Contain hundreds of thousands of genes, each of which is a specific region of the DNA molecule, or locus

4. Human Life Cycle Each somatic cell (body cell) has 46 chromosomes or 23 matching pairs (diploid) Karyotype: male Autosomes: non- sex chromosomes Sex chromosomes: determine gender (XX; XY)

5. Human Life Cycle Gametes (sex cells) have a single set of 22 autosomes and a single sex chromosome, either X or Y With 23 chromosomes, they are haploid haploid number: n = 23 diploid number: 2n = 46 Haploid sperm + haploid ova zygote (2n) fertilization 2n2n n n

6. Meiosis Reduces chromosome number from diploid to haploid Increases genetic variation among offspring Steps resemble steps in mitosis Single replication of DNA is followed by 2 consecutive cell divisions Meiosis I Meiosis II Produces 4 different daughter cells which have half the number of chromosomes as the original cell

7. In the first division, meiosis I, homologous chromosomes are paired While they are paired, they cross over and exchange genetic information The homologous pairs are then separated, and two daughter cells are produced

9. Interphase I Chromosomes replicate (still as chromatin) Duplicated chromosomes consist of 2 identical sister chromatids attached by centromere Centriole pairs replicate

10. Figure 8.14, part 1 MEIOSIS I : Homologous chromosomes separate INTERPHASEPROPHASE I METAPHASE I ANAPHASE I Centrosomes (with centriole pairs) Nuclear envelope Chromatin Sites of crossing over Spindle Sister chromatids Tetrad Microtubules attached to kinetochore Metaphase plate Centromere (with kinetochore) Sister chromatids remain attached Homologous chromosomes separate Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

11. Meiosis I This cell division separates the 2 chromosomes of each homologous pair and reduce the chromosome number by one-half

12. Prophase I Chromosomes condense Synapsis occurs (homologues pair) Chromosomes seen as distinct structures; each chromosome has 2 chromatids, so each synapsis forms a tetrad

13. Prophase I Sister chromatids held together by centromeres; non- sister chromatids held together by chiasmata where crossing-over occurs (exchange of DNA)

15. Late Prophase I Centriole pairs move apart and spindle fibers form Nuclear envelope disappears and nucleoli disperse

16. Prophase I

17. Metaphase I Homologous chromosomes line up along metaphase plate

18. Metaphase I

19. Anaphase I Homologous chromosomes separate, independently from others

20. Anaphase I

21. Telophase I and Cytokinesis Each pole now has a haploid set of chromosomes (each with 2 sister chromatids) Usually, cytokinesis occurs simultaneously with telophase I, forming 2 haploid daughter cells (cleavage furrow forms in animals; cell plate forms in plants)

22. Telophase I

23. Meiosis II is essentially the same as mitosis The sister chromatids of each chromosome separate The result is four haploid daughter cells

24. Figure 8.14, part 2 MEIOSIS II : Sister chromatids separate TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavage furrow Sister chromatids separate TELOPHASE II AND CYTOKINESIS Haploid daughter cells forming Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

25. Meiosis II This cell division separates the 2 sister chromatids of each chromosome

26. Prophase II Spindle apparatus forms and chromosomes move toward metaphase II plate

27. Prophase II

29. Metaphase II Chromosomes align singly on the metaphase plate

30. Metaphase II

31. Anaphase II Sister chromatids of each pair (now individual chromosomes) separate and move toward opposite poles of the cell

32. Anaphase II

34. Telophase II and Cytokinesis Nuclei form at opposite poles of the cell Cytokinesis occurs producing 4 haploid daughter cells (each genetically different)

35. Telophase II

37. Key Differences Between Mitosis and Meiosis Meiosis is a reduction division Mitotic cells produce clones (same xsome #) Meiosis produces haploid cells Meiosis creates genetic variation Mitosis produces 2 identical daughter cells Meiosis produces 4 genetically different daughter cells Meiosis is 2 successive nuclear divisions Mitosis has one division

38. Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings

39. Spermatogenesis Process of sperm production Results in 4 viable sperm

41. Oogenesis Process of egg (ova) production Results in 1 viable egg and 3 polar bodies that will not survive Polar bodies result from an uneven division of cytoplasm

43. Mechanisms of Genetic Variation Independent assortment—each pair of homologous chromosomes separate independently Results in gametes with different gene combinations Crossing-over—exchange of genetic material between non-sister chromatids Results in genetic recombination Random fertilization—random joining of two gametes

44. Independent Assortment

45. Figure 8.16 POSSIBILITY 1POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1Combination 2Combination 3Combination 4 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

46. Mechanisms of Genetic Variation Independent assortment—each pair of homologous chromosomes separate independently Results in gametes with different gene combinations Crossing-over—exchange of genetic material between non-sister chromatids Results in genetic recombination Random fertilization—random joining of two gametes

47. Crossing over is the exchange of corresponding segments between two homologous chromosomes Genetic recombination results from crossing over during prophase I of meiosis This increases variation further Crossing over further increases genetic variability

48. Somatic cells of each species contain a specific number of chromosomes Human cells have 46, making up 23 pairs of homologous chromosomes MEIOSIS AND CROSSING OVER Chromosomes are matched in homologous pairs Chromosomes Centromere Sister chromatids Figure 8.12 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

49. Figure 8.18A Tetrad Chaisma Centromere Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

50. Figure 8.17A, B Coat-color genesEye-color genes BrownBlack CE ce WhitePink CE ce CE ce Tetrad in parent cell (homologous pair of duplicated chromosomes) Chromosomes of the four gametes Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

51. How crossing over leads to genetic recombination Figure 8.18B Tetrad (homologous pair of chromosomes in synapsis) Breakage of homologous chromatids Joining of homologous chromatids Chiasma Separation of homologous chromosomes at anaphase I Separation of chromatids at anaphase II and completion of meiosis Parental type of chromosome Recombinant chromosome Parental type of chromosome Gametes of four genetic types 1 2 3 4 Coat-color genes Eye-color genes Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

52. Crossing Over In Prophase I of Meiosis I, synapsis occurs allowing the crossing over of genetic material between non-sister chromatids Creates new combinations of genes not seen in either parent

53. Mechanisms of Genetic Variation Independent assortment—each pair of homologous chromosomes separate independently Results in gametes with different gene combinations Crossing-over—exchange of genetic material between non-sister chromatids Results in genetic recombination Random fertilization—random joining of two gametes

54. Random Fertilization Random as to which gametes join and form a gamete

55. Importance of Genetic Variation Essential to evolution (change over time) Variation can cause changes that leads to different traits Some favorable Some unfavorable

56. Errors and Exceptions in Chromosomal Inheritance Alterations in chromosome number or structure causes some genetic disorders Physical and chemical disturbances Errors during meiosis

57. To study human chromosomes microscopically, researchers stain and display them as a karyotype A karyotype usually shows 22 pairs of autosomes and one pair of sex chromosomes ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE

58. Preparation of a karyotype Figure 8.19 Blood culture 1 Centrifuge Packed red And white blood cells Fluid 2 Hypotonic solution 3 Fixative White Blood cells Stain 45 Centromere Sister chromatids Pair of homologous chromosomes Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

59. Human female bands Figure 8.19x1 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

60. Human female karyotype Figure 8.19x2 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

61. Human male bands Figure 8.19x3 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

62. Human male karyotype Figure 8.19x4 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

63. Alterations of Chromosome Numbers Nondisjunction—pair of homologues do not move apart during Meiosis I, or sister chromatids do not separate during Meiosis II Results in uneven distribution of chromosomes to daughter cells

64. Alterations of Chromosome Numbers Aneuploidy: abnormal chromosome number Trisomy: three copies of chromosomes Monosomy: one copy of a chromosome Trisomy and monosomy are usually lethal

65. Nondisjunction Copyright © 2000Pearson Education, Inc. publishing Benjamin Cummings

66. Abnormal chromosome count is a result of nondisjunction Either homologous pairs fail to separate during meiosis I Accidents during meiosis can alter chromosome number Figure 8.21A Nondisjunction in meiosis I Normal meiosis II Gametes n + 1 n – 1 Number of chromosomes Copyright © 2003Pearson Education, Inc. publishing Benjamin Cummings

67. Or sister chromatids fail to separate during meiosis II Figure 8.21B Normal meiosis I Nondisjunction in meiosis II Gametes n + 1n – 1nn Number of chromosomes Copyright © 2003Pearson Education, Inc. publishing Benjamin Cummings

68. Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome Figure 8.21C Egg cell Sperm cell n + 1 n (normal) Zygote 2n + 1 Copyright © 2003Pearson Education, Inc. publishing Benjamin Cummings

69. Trisomy 21 (Down Syndrome) *Short stature, characteristic facial features, and heart defects (varying severity) *Most common serious birth defect (1 out of 700 births) *Mothers 35+ years of age have higher chance of having a Down baby

70. The chance of having a Down syndrome child goes up with maternal age Figure 8.20C Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

71. Down syndrome karyotype Figure 8.20Ax Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

72. Nondisjunction with Sex Chromosomes

73. Table 8.22 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

74. Klinefelter’s karyotype Figure 8.22Ax Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

75. XYY karyotype Figure 8.22x

76. Breakage of a chromosome can lead to four types of changes in chromosome structure Deletion: chromosomal fragment is lost during cell division Duplication: fragment may join to the homologous chromosome Inversion: fragment may reattach to the original chromosome but in the reverse orientation Translocation: fragment joins a nonhomologous chromosome Alterations of Chromosome Structure

77. Chromosome Mutation: Deletion Deleted region Before Deletion After Deletion

78. Cri du Chat Syndrome: Partial deletion 5p

79. Before inversion After inversion Inverted region Chromosome Mutation: Inversion

80. Chromosome 4 Chromosome 20 Region being moved Before TranslocationAfter Translocation Chromosome Mutation: Translocation

81. Chromosomal changes in a somatic cell can cause cancer Figure 8.23C Chromosome 9 A chromosomal translocation in the bone marrow is associated with chronic myelogenous leukemia Chromosome 22 Reciprocal translocation “Philadelphia chromosome” Activated cancer-causing gene Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

82. Philadelphia Chromosome t(9,22)

83. Translocation Figure 8.23Bx Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

84. Gametes MeiosisFertilization Zygote Mitosis Diploid multicellular organism n 2n2n nn 2n2n Animal Life Cycle

85. Fungi and Algae Life Cycle Gametes Meiosis Fertilization Zygote Mitosis Haploid multicellular organism n 2n2n n n n n

86. Alternation of Generations Life Cycles

87. Acknowledgements Unless otherwise noted, illustrations are credited to Pearson Education which have been borrowed from BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2003. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher. BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2001. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.