1. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint ® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 13 Meiosis and Sexual Life Cycles

2. Overview: Variations on a Theme Living organisms are distinguished by their ability to reproduce their own kind Heredity is the transmission of traits from one generation to the next Variation is demonstrated by the differences in appearance that offspring show from parents and siblings Genetics is the scientific study of heredity and variation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

3. Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes In a literal sense, children do not inherit particular physical traits from their parents It is genes that are actually inherited Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

4. Inheritance of Genes Genes are the units of heredity, and are made up of segments of DNA Genes are passed to the next generation through reproductive cells called gametes (sperm and eggs) Each gene has a specific location called a locus on a certain chromosome Most DNA is packaged into chromosomes One set of chromosomes is inherited from each parent Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

5. Comparison of Asexual and Sexual Reproduction In asexual reproduction, one parent produces genetically identical offspring by mitosis Asexual reproduction gives rise to clones A clone is a group of genetically identical individuals from the same parent Prokaryotes and single celled eukaryotes reproduce this way Some multicellular eukaryotes as well Video: Hydra Budding Video: Hydra Budding Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

6. Fig. 13-2 (a) Hydra (b) Redwoods Parent Bud 0.5 mm

7. Comparison of Asexual and Sexual Reproduction In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents Heredity – genes from parents Variation – unique combinations The key to the variability is the behavior of chromosomes during sexual life cycle Video: Hydra Budding Video: Hydra Budding Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

8. Concept 13.2: Fertilization and meiosis alternate in sexual life cycles A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism from conception to production of offspring Chromosomes in gametes (reproductive cells) behave differently than in somatic cells That is the focus of this chapter Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

9. Sets of Chromosomes in Human Cells Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes A karyotype is an ordered display of the pairs of chromosomes from a cell The two chromosomes in each pair are called homologous chromosomes, or homologs Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

10. Fig. 13-3 APPLICATION TECHNIQUE Pair of homologous replicated chromosomes 5 µm Centromere Sister chromatids Metaphase chromosome

11. The sex chromosomes are called X and Y Human females have a homologous pair of X chromosomes (XX) 23 homologous pairs Human males have one X and one Y chromosome 22 homologous pairs and 1 XY pair The 22 pairs of chromosomes that do not determine sex are called autosomes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sets of Chromosomes in Human Cells

12. Each pair of homologous chromosomes includes one chromosome from each parent The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father A cell with two sets of chromosomes is called A diploid cell and is designated (2n) For humans, the diploid number is 46 (2n = 46) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sets of Chromosomes in Human Cells

13. In a cell in which DNA synthesis has occurred, each chromosome is replicated Each replicated chromosome consists of two identical sister chromatids Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sets of Chromosomes in Human Cells

14. Fig. 13-4 Key Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) 2n = 6 Centromere Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

15. A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n) For humans, the haploid number is 23 (n = 23) Each set of 23 consists of 22 autosomes and a single sex chromosome In an unfertilized egg (ovum), the sex chromosome is X In a sperm cell, the sex chromosome may be either X or Y Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sets of Chromosomes in Human Cells

16. Fertilization is the union of gametes (the sperm and the egg) Both haploid The fertilized egg is called a zygote and has one set of chromosomes from each parent Now diploid The zygote produces somatic cells by mitosis and develops into an adult Behavior of Chromosome Sets in the Human Life Cycle Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

17. At sexual maturity, the ovaries and testes produce haploid gametes Gametes are the only types of human cells produced by meiosis, rather than mitosis Meiosis results in one set of chromosomes in each gamete Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Behavior of Chromosome Sets in the Human Life Cycle

18. Fig. 13-5 Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Egg (n) Sperm (n) MEIOSISFERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

19. The Variety of Sexual Life Cycles The alternation of meiosis and fertilization is common to all organisms that reproduce sexually Why do gametes have to be haploid? The three main types of sexual life cycles differ in the timing of meiosis and fertilization Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

20. In animals, meiosis produces gametes, which undergo no further cell division before fertilization Gametes are the only haploid cells in animals Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Variety of Sexual Life Cycles

21. Fig. 13-6a Key Haploid (n) Diploid (2n) Gametes n n n 2n2n2n2n Zygote MEIOSISFERTILIZATION Mitosis Diploid multicellular organism (a) Animals

22. Plants and some algae exhibit an alternation of generations This life cycle includes both a diploid and haploid multicellular stage The diploid organism, called the sporophyte, makes haploid spores by meiosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Variety of Sexual Life Cycles

23. Each spore grows by mitosis into a haploid organism called a gametophyte A gametophyte makes haploid gametes by mitosis These plants don’t have gonads – they make a whole separate plant to make gametes Fertilization of gametes results in a diploid sporophyte Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Variety of Sexual Life Cycles

24. Fig. 13-6b Key Haploid (n) Diploid (2n) n n n n n 2n2n 2n2n Mitosis Zygote Spores Gametes MEIOSISFERTILIZATION Diploid multicellular organism (sporophyte) Haploid multi- cellular organism (gametophyte) (b) Plants and some algae

25. In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage The zygote produces haploid cells by meiosis Each haploid cell grows by mitosis into a haploid multicellular organism The haploid adult produces gametes by mitosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Variety of Sexual Life Cycles

26. Fig. 13-6c Key Haploid (n) Diploid (2n) Mitosis Gametes Zygote Haploid unicellular or multicellular organism MEIOSIS FERTILIZATION n n n n n 2n2n (c) Most fungi and some protists

27. Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis However, only diploid cells can undergo meiosis In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Variety of Sexual Life Cycles

28. Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid Like mitosis, meiosis is preceded by the replication of chromosomes Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis Each daughter cell has only half as many chromosomes as the parent cell Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

29. The Stages of Meiosis In the first cell division (meiosis I), homologous chromosomes separate Meiosis I results in two haploid daughter cells with replicated chromosomes; it is called the reductional division In the second cell division (meiosis II), sister chromatids separate Meiosis II results in four haploid daughter cells with unreplicated chromosomes; it is called the equational division Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

30. Fig. 13-7-1 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes

31. Fig. 13-7-2 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I Homologous chromosomes separate 1 Haploid cells with replicated chromosomes

32. Fig. 13-7-3 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I Homologous chromosomes separate 1 Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes

33. Meiosis I is preceded by interphase, in which chromosomes are replicated to form sister chromatids The sister chromatids are genetically identical and joined at the centromere The single centrosome replicates, forming two centrosomes BioFlix: Meiosis BioFlix: Meiosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Stages of Meiosis

34. Fig. 13-8 Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Prophase II Metaphase IIAnaphase II Telophase II and Cytokinesis Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Homologous chromosomes Fragments of nuclear envelope Centromere (with kinetochore) Metaphase plate Microtubule attached to kinetochore Sister chromatids remain attached Homologous chromosomes separate Cleavage furrow Sister chromatids separate Haploid daughter cells forming

35. Division in meiosis I occurs in four phases: –Prophase I –Metaphase I –Anaphase I –Telophase I and cytokinesis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Stages of Meiosis

36. Metaphase I Fig. 13-8a Prophase IAnaphase I Telophase I and Cytokinesis Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Homologous chromosomes Fragments of nuclear envelope Centromere (with kinetochore) Metaphase plate Microtubule attached to kinetochore Sister chromatids remain attached Homologous chromosomes separate Cleavage furrow

37. Prophase I Prophase I typically occupies more than 90% of the time required for meiosis Chromosomes begin to condense In synapsis, homologous chromosomes loosely pair up, aligned gene by gene Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Stages of Meiosis

38. Each pair of chromosomes forms a tetrad, a group of four chromatids Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred In crossing over, nonsister chromatids exchange DNA segments Increases variability in offspring Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Stages of Meiosis

39. Fig. 13-8b Prophase IMetaphase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore

40. Metaphase I In metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad Microtubules from the other pole are attached to the kinetochore of the other chromosome Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Stages of Meiosis

41. Fig. 13-8b Prophase IMetaphase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore

42. Anaphase I In anaphase I, pairs of homologous chromosomes separate One chromosome moves toward each pole, guided by the spindle apparatus Sister chromatids remain attached at the centromere and move as one unit toward the pole Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Stages of Meiosis

43. Telophase I and Cytokinesis In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids Cytokinesis usually occurs simultaneously, forming two haploid daughter cells In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Stages of Meiosis

44. Fig. 13-8c Anaphase I Telophase I and Cytokinesis Sister chromatids remain attached Homologous chromosomes separate Cleavage furrow

45. How do the sister chromatids stay together during Meiosis I Proteins called cohesins “glue” them together Also present during Mitosis At Anaphase I cleaved so that homologs can separate At anaphase II cleaved so that sister chromatids can separate Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Stages of Meiosis

46. No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Stages of Meiosis

47. Division in meiosis II also occurs in four phases: –Prophase II –Metaphase II –Anaphase II –Telophase II and cytokinesis Meiosis II is very similar to mitosis equational division Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Stages of Meiosis

48. Fig. 13-8d Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming

49. Prophase II In prophase II, a spindle apparatus forms In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Stages of Meiosis

50. Metaphase II In metaphase II, the sister chromatids are arranged at the metaphase plate Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical The kinetochores of sister chromatids attach to microtubules extending from opposite poles Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Stages of Meiosis

51. Fig. 13-8e Prophase IIMetaphase II Two different colors on the sister chromatids where they have crossed over

52. Anaphase II In anaphase II, the sister chromatids separate The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Stages of Meiosis

53. Telophase II and Cytokinesis In telophase II, the chromosomes arrive at opposite poles Nuclei form, and the chromosomes begin decondensing Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Stages of Meiosis

54. Cytokinesis separates the cytoplasm At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes Each daughter cell is genetically distinct from the others and from the parent cell Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Stages of Meiosis

55. Fig. 13-8f Anaphase II Telephase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming

56. A Comparison of Mitosis and Meiosis Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

57. Fig. 13-9a MITOSIS MEIOSIS MEIOSIS I Prophase I Chiasma Chromosome replication Homologous chromosome pair Chromosome replication 2n = 6 Parent cell Prophase Replicated chromosome Metaphase Metaphase I Anaphase I Telophase I Haploid n = 3 Daughter cells of meiosis I MEIOSIS II Daughter cells of meiosis II n n n n 2n2n 2n2n Daughter cells of mitosis Anaphase Telophase

58. Fig. 13-9b SUMMARY Meiosis Mitosis Property DNA replication Number of divisions Occurs during interphase before mitosis begins One, including prophase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes Does not occur Number of daughter cells and genetic composition Two, each diploid (2n) and genetically identical to the parent cell Role in the animal body Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Occurs during interphase before meiosis I begins Two, each including prophase, metaphase, anaphase, and telophase Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes

59. Three events are unique to meiosis, and all three occur in meiosis l: –Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information –At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes –At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings A Comparison of Mitosis and Meiosis

60. Sister chromatid cohesion allows sister chromatids of a single chromosome to stay together through meiosis I Protein complexes called cohesins are responsible for this cohesion In mitosis, cohesins are cleaved at the end of metaphase In meiosis, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II (separation of sister chromatids) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings A Comparison of Mitosis and Meiosis

61. Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution Mutations (changes in an organism’s DNA) are the original source of genetic diversity Mutations create different versions of genes called alleles Reshuffling of alleles during sexual reproduction produces genetic variation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

62. Origins of Genetic Variation Among Offspring The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation Three mechanisms contribute to genetic variation: – Independent assortment of chromosomes – Crossing over – Random fertilization Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

63. Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

64. The number of combinations possible when chromosomes assort independently into gametes is 2 n, where n is the haploid number For humans (n = 23), there are more than 8 million (2 23 ) possible combinations of chromosomes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Independent Assortment of Chromosomes

65. Fig. 13-11-1 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I

66. Fig. 13-11-2 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II

67. Fig. 13-11-3 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1Combination 2Combination 3Combination 4

68. Crossing Over Crossing over produces recombinant chromosomes, which combine genes inherited from each parent Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

69. In crossing over, homologous portions of two nonsister chromatids trade places Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Crossing Over

70. Fig. 13-12-1 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis

71. Fig. 13-12-2 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere TEM

72. Fig. 13-12-3 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere Anaphase I TEM

73. Fig. 13-12-4 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere Anaphase I Anaphase II TEM

74. Fig. 13-12-5 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere Anaphase I Anaphase II Daughter cells Recombinant chromosomes TEM In meiosis I this homologous pairs separate Crossing over actually mixes up the genes on the paternal and maternal chromosome before this happens Crossing over exchanges different versions of the same gene (called alleles) Get a lot more variability this way than just sorting out sister chromatids

75. Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

76. The Evolutionary Significance of Genetic Variation Within Populations Natural selection results in the accumulation of genetic variations favored by the environment Sexual reproduction contributes to the genetic variation in a population, which originates from mutations Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

77. You should now be able to: 1.Distinguish between the following terms: somatic cell and gamete; autosome and sex chromosomes; haploid and diploid 2.Describe the events that characterize each phase of meiosis 3.Describe three events that occur during meiosis I but not mitosis 4.Name and explain the three events that contribute to genetic variation in sexually reproducing organisms Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings