1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 13 Meiosis and Sexual Life Cycles

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: Hereditary Similarity and Variation 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 shows that offspring differ in appearance from parents and siblings Genetics is the scientific study of heredity and variation

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

4. 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

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance of Genes Genes are the units of heredity Genes are segments of DNA Each gene has a specific locus (place) on a certain chromosome You inherit one set of chromosomes from each parent Reproductive cells called gametes (sperm and eggs) unite, passing genes to the next generation

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Comparison of Asexual and Sexual Reproduction In asexual reproduction, one parent produces genetically identical offspring by mitosis In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents

7. LE 13-2 Parent 0.5 mm Bud

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sets of Chromosomes in Human Cells Each human somatic cell (any cell other than a gamete) has 46 chromosomes arranged in pairs 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 homologues Homologous chromosomes in a pair carry genes controlling the same inherited characteristics

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE What is a karyotype? A karyotype is a photograph of an individual’s chromosomes arranged in order A karyotype is a picture, a display of the 46 chromosomes. Shows 23 pairs of chromosomes, each pair with the same length, centromere position, and staining pattern.

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Blood culture Fluid Centrifuge Packed red and white blood cells Hypotonic solution Fixative White blood cells Stain Centromere Pair of homologous chromosomes Sister chromosomes 2,600X A blood culture is centrifuged to separate the blood cells from the culture fluid. 1 The fluid is discarded, and a hypotonic solution is mixed with the cells. This makes the red blood cells burst. The white blood cells swell but do not burst, and their chromosomes spread out. 2 Another centrifugation step separates the swollen white blood cells. The fluid containing the remnants of the red blood cells is poured off. A fixative (preservative) is mixed with the white blood cells. A drop of the cell suspension is spread on a microscope slide, dried, and stained. 3 The slide is viewed with a microscope equipped with a digital camera. A photograph of the chromosomes is entered into a computer, which electronically arranges them by size and shape. 4 The resulting display is the karyotype. The 46 chromosomes here include 22 pair of autosomes and 2 sex chromosomes, X and Y. Although difficult to discern in the karyotype, each of the chromosomes consists of two sister chromatids lying very close together (see diagram). 5 – Preparation of a karyotype from a blood sample Figure 8.19

12. LE 13-3 5 µm Pair of homologous chromosomes Sister chromatids Centromere

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Karyotypes Go to activities (on left hand side) and click on “karyotyping” http://www.biology.arizona.edu/cell_bio/cell_bio.ht mlhttp://www.biology.arizona.edu/cell_bio/cell_bio.ht ml http://www.biology.arizona.edu/cell_bio/cell_bio.ht mlhttp://www.biology.arizona.edu/cell_bio/cell_bio.ht ml

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sex chromosome pair Male pair is: XY Female pair is: XX How is sex determined?

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The sex chromosomes are called X and Y Human females have a homologous pair of X chromosomes (XX) Human males (XY) have one X and one Y chromosome The 22 pairs of chromosomes that do not determine sex are called autosomes

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 The number of chromosomes in a single set is represented by n A cell with two sets is called diploid (2n) For humans, the diploid number is 46 (2n = 46)

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In a cell in which DNA synthesis has occurred, each chromosome is replicated Each replicated chromosome consists of two identical sister chromatids

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

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gametes are haploid cells, containing only one set of chromosomes 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

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Behavior of Chromosome Sets in the Human Life Cycle 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, the fusing of gametes, restores the diploid condition, forming a zygote The diploid zygote develops into an adult

21. LE 13-5 Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm cell (n) Testis Ovary Mitosis and development Multicellular diploid adults (2n = 46) FERTILIZATIONMEIOSIS Diploid zygote (2n = 46)

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Variety of Sexual Life Cycles The alternation of meiosis and fertilization is common to all organisms that reproduce sexually The three main types of sexual life cycles differ in the timing of meiosis and fertilization

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

24. LE 13-6 Key Haploid Diploid Gametes n Diploid multicellular organism (sporophyte) Mitosis Diploid multicellular organism FERTILIZATION MEIOSIS Zygote n n 2n2n 2n2n Animals Plants and some algae Most fungi and some protists n n n n n n n n n n FERTILIZATION MEIOSIS Gametes Zygote Mitosis 2n2n 2n2n 2n2n Spores Haploid multicellular organism (gametophyte) Haploid multicellular organism

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plants and some algae exhibit an alternation of generations This life cycle includes two multicellular generations or stages: one diploid and one haploid The sporophyte is diploid, the sporophyte, makes haploid spores by meiosis The gametophyte is haploid. Each spore grows by mitosis into a haploid organism called a gametophyte A gametophyte makes haploid gametes by mitosis

26. LE 13-6b Key Haploid Diploid multicellular organism (sporophyte) Plants and some algae n n n n n FERTILIZATION MEIOSIS Gametes Zygote Mitosis 2n2n 2n2n Spores Haploid multicellular organism (gametophyte)

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

28. LE 13-6c Key Haploid Diploid Most fungi and some protists n n n n n FERTILIZATION MEIOSIS Gametes Zygote Mitosis 2n2n Haploid multicellular organism

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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, chromosome halving and doubling contribute to genetic variation in offspring

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Stages of Meiosis In meiosis I, homologous chromosomes separate Meiosis I results in two haploid daughter cells with replicated chromosomes In the second cell division (meiosis II), sister chromatids separate Meiosis II results in four haploid daughter cells with unreplicated chromosomes

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

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

34. LE 13-8aa Centrosomes (with centriole pairs) Nuclear envelope Chromatin Chromosomes duplicate INTERPHASE MEIOSIS I: Separates homologous chromosomes METAPHASE I ANAPHASE I

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Division in meiosis I occurs in four phases: Prophase I Metaphase I Anaphase I Telophase I

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Prophase I Prophase I typically occupies more than 90% of the time required for meiosis Chromosomes begin to condense In synapsis, homologous chromosomes pair up, aligned gene by gene In crossing over, nonsister chromatids exchange DNA segments Each pair of chromosomes forms a tetrad, a group of four chromatids Each tetrad has one or more chiasmata, X-shaped regions where crossing over occurred

37. LE 13-8ab Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Pairs of homologous chromosomes split up Tetrads line up

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Metaphase I At 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

39. LE 13-8ab Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Pairs of homologous chromosomes split up Tetrads line up

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

41. LE 13-8ab Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Pairs of homologous chromosomes split up Tetrads line up

42. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Telophase I and Cytokinesis In 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 No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

43. LE 13-8b Cleavage furrow MEIOSIS II: Separates sister chromatids PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Prophase II Meiosis II is very similar to mitosis In prophase II, a spindle apparatus forms In late prophase II (not shown in the art), chromosomes (each still composed of two chromatids) move toward the metaphase plate

45. LE 13-8b Cleavage furrow MEIOSIS II: Separates sister chromatids PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Metaphase II At 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

47. LE 13-8b Cleavage furrow MEIOSIS II: Separates sister chromatids PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Anaphase II At anaphase II, the sister chromatids separate The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles

49. LE 13-8b Cleavage furrow MEIOSIS II: Separates sister chromatids PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

50. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Telophase II and Cytokinesis In telophase II, the chromosomes arrive at opposite poles Nuclei form, and the chromosomes begin decondensing 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

51. LE 13-8b Cleavage furrow MEIOSIS II: Separates sister chromatids PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

53. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 and are carried to opposite poles of the cell

54. LE 13-9 Propase Duplicated chromosome (two sister chromatids) Chromosome replication 2n = 6 Parent cell (before chromosome replication) Chromosome replication MITOSISMEIOSIS Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Tetrads positioned at the metaphase plate Metaphase I Chromosomes positioned at the metaphase plate Metaphase Anaphase Telophase Homologues separate during anaphase I; sister chromatids remain together Sister chromatids separate during anaphase Daughter cells of meiosis I Haploid n = 3 Anaphase I Telophase I MEIOSIS II Daughter cells of mitosis 2n2n 2n2n n Sister chromatids separate during anaphase II n nn Daughter cells of meiosis II

55. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PropertyMitosisMeiosis DNA replication During interphase DivisionsOneTwo Synapsis and crossing over Do not occurForm tetrads in prophase I Daughter cells, genetic composition Two diploid, identical to parent cell Four haploid, different from parent cell and each other Role in animal body Produces cells for growth and tissue repair Produces gametes

56. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Reshuffling of different versions of genes during sexual reproduction produces genetic variation

57. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

58. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 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

59. LE 13-10 Key Maternal set of chromosomes Paternal set of chromosomes Possibility 1 Possibility 2 Combination 2 Combination 1 Combination 3 Combination 4 Daughter cells Metaphase II Two equally probable arrangements of chromosomes at metaphase I

60. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Crossing Over Crossing over produces recombinant chromosomes, which combine genes inherited from each parent Crossing over in prophase I, as homologous chromosomes pair up gene by gene homologous portions of two nonsister chromatids trade places Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome

61. LE 13-11 Prophase I of meiosis Tetrad Nonsister chromatids Chiasma, site of crossing over Recombinant chromosomes Metaphase I Metaphase II Daughter cells

62. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations Crossing over adds even more variation Each zygote has a unique genetic identity

63. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evolutionary Significance of Genetic Variation Within Populations Natural selection results in accumulation of genetic variations favored by the environment Sexual reproduction contributes to the genetic variation in a population, which ultimately results from mutations

64. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Results of Mitosis and Meiosis Mitosis – Two diploid cells produced – Each identical to parent Meiosis – Four haploid cells produced – Differ from parent and one another

65. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis compared to mitosis Mitosis In somatic cells One duplication of chromosomes followed by one division ( duplication takes place during “S”phase before mitosis starts) Results in two daughter cells identical to the parent cell and to each other Meiosis In sex cells One duplication of chromosomes followed by two divisions ( duplication takes place during “S”phase before meiosis starts) Results in four daughter cells with half the number of chromosomes. Variety or new combinations result.

66. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings MITOSIS AND MEIOSIS COMPARED Occurs in somatic cells One duplication one division Result in two diploid (2N) daughter cells Daughter cells are identical to parent cell Occurs in gametes only One duplication two divisions Result in four haploid (N) daughter cells Daughter cells are different to parent cell. Introduces variety.

67. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Web sites to check http://www.pbs.org/wgbh/nova/baby/divi_flash. htmlhttp://www.pbs.org/wgbh/nova/baby/divi_flash. html http://highered.mcgraw- hill.com/sites/0072437316/student_view0/chapt er12/animations.htmlhttp://highered.mcgraw- hill.com/sites/0072437316/student_view0/chapt er12/animations.html