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Meiosis Bio 11 Tuesday, October 7, 2008. Which one of the following does not occur during mitotic anaphase? A) The centromeres of each chromosome divide.

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Presentation on theme: "Meiosis Bio 11 Tuesday, October 7, 2008. Which one of the following does not occur during mitotic anaphase? A) The centromeres of each chromosome divide."— Presentation transcript:

1 Meiosis Bio 11 Tuesday, October 7, 2008

2 Which one of the following does not occur during mitotic anaphase? A) The centromeres of each chromosome divide. B) Sister chromatids separate. C) The chromatid DNA replicates. D) Daughter chromosomes begin to move toward opposite poles of the cell. E) All of the choices occur during mitotic anaphase.

3 Asexual Reproduction is a primary means of propagation for many species

4 Sexual reproduction generates diversity Variety in offspring improves chances of adaptation to changing environments

5 The offspring of sexual reproduction inherit parental traits Offspring inherit DNA from each parent Offspring are not the same DNA is from the same source

6 LE 8-13 Haploid gametes (n  23) Egg cell Sperm cell FertilizationMeiosis Diploid zygote (2n  46) n Multicellular diploid adults (2n  46) Mitosis and development 2n n Meiosis is the process of cell division that allows gamete formation Gametes are haploid (n), somatic cells are diploid (2n)

7 Chromosomes come in pairs For diploid organims (2n) One chromosome in a paired set comes from each parent #’s 1-22 are autosomes X and Y are sex chromosomes

8 LE 8-12 Homologous Chromosomes Centromere Sister chromatids

9 Homologous chromosomes Pairs of chromosomes are homologous The site for particular genes are called loci (singular: locus) Identical strands of the same chromosome are sister chromatids Loci 

10 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

11 Overview of Meiosis Meiosis consists of 2 major stages, Meiosis I and Meiosis II Cell copies DNA once, divides twice, creating 4 haploid cells In meiosis I, homologous pairs of chromosomes are grouped, recombined, and then segregated into two intermediate cells In meiosis II, those cells are divided

12 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

13 LE 8-14a I NTERPHASE P ROPHASE  M ETAPHASE  A NAPHASE  M EIOSIS  Centrosomes (with centriole pairs) Sites of crossing over Spindle Microtubules attached to kinetochore Metaphase plate Sister chromatids remain attached Homologous chromosomes separate Centromere (with kinetochore) Tetrad Sister chromatids Chromatin Nuclear envelope : Homologous chromosome separate

14 LE 13-8aa Centrosomes (with centriole pairs) Nuclear envelope Chromatin Chromosomes duplicate INTERPHASE MEIOSIS I: Separates homologous chromosomes METAPHASE I ANAPHASE I (Interphase precedes meiosis, of course)

15 Division in meiosis I occurs in four phases: Prophase I Metaphase I Anaphase I Telophase I

16 Prophase I >90% of the time required for meiosis Chromosomes condense Synapsis: homologous chromosomes pair up Crossing over: nonsister chromatids exchange DNA segments Tetrad: four chromatids Chiasmata: X-shaped regions where crossing over occurred

17 Crossing-Over in Prophase I

18 LE 8-18a Tetrad Chiasma Centromere TEM 2,200 

19 Crossing-over in Prophase I

20 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

21 Metaphase I Tetrads line up at the metaphase plate Microtubules from one pole are attached Microtubules from the other pole are attached to the kinetochore of the other chromosome Animation: Metaphase I Animation: Metaphase I

22 Independent Assortment in Metaphase I Random alignment of maternal/paternal chromosomes at the metaphase plate Produces genetic variability within populations

23 Independent Assortment in Metaphase I

24

25 Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis 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- 1, where n is the haploid number For humans (n = 23), there are more than 8 million (2 22 ) possible combinations of chromosomes

26 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

27 Anaphase I homologous chromosomes separate One chromosome moves toward each pole Sister chromatids remain attached at the centromere

28 Meiosis II Resembles mitosis in its mechanisms However, only half the chromosomes are present in meiosis II compared with mitosis

29 LE 8-14b Cleavage furrow T ELOPHASE  P ROPHASE  M ETAPHASE  A NAPHASE  T ELOPHASE  Sister chromatids separate Haploid daughter cells forming M EIOSIS  : Sister chromatids separate  AND C YTOKINESIS

30 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

31 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 Animation: Metaphase II Animation: Metaphase II

32 Spindle Microtubules bind to the Kinetochore Prometaphase marks the binding of spindle fibers to kinetochore Chromosomes will be pulled apart in anaphase

33 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

34 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 Animation: Anaphase II Animation: Anaphase II

35 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

36 Which one of the following statements is false? A) A typical body cell is called a somatic cell. B) Gametes are haploid cells. C) Somatic cells are diploid. D) Gametes are made by mitosis. E) A zygote is a fertilized egg.

37 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 Animation: Telophase II and Cytokinesis Animation: Telophase II and Cytokinesis

38 LE 8-21a Number of chromosomes Gametes Normal meiosis  Normal meiosis  Nondisjunction in meiosis  n  1 n  1

39 LE 8-21b Number of chromosomes Gametes Normal meiosis  Nondisjunction in meiosis  n  1 n  1 n n

40 LE 8-21c Egg cell Sperm cell n (normal) n  1 Zygote 2n  1

41 Downs syndrome is caused by a nondisjunction (trisomy) at chromosome 21

42 LE 8-20c Infants with Down syndrome (per 1,000 births) Age of mother

43 Figure 8.20A

44 LE 8-19 Packed red and white blood cells Hypotonic solution Centrifuge Blood culture Fluid Fixative White blood cells Stain Centromere Siste r chromatids Pair of homologous chromosomes 2,600 

45 Comparing mitosis and meiosis

46

47 PropertyMitosisMeiosis DNA replication Divisions Synapsis and crossing over Daughter cells, genetic composition Role in animal body

48 PropertyMitosisMeiosis DNA replication During interphase DivisionsOneTwo Synapsis and crossing over Do not occurIn 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

49 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

50 Review of Meiosis

51 Spermatogenesis vs. Oogenesis

52 Meiosis One spermatocyte produces 4 sperm One oocyte produces 1 viable egg and 3 non-viable polar bodies

53

54 Variation from sexual reproduction contributes to evolution Reshuffling of different versions of genes (alleles) during sexual reproduction produces genetic variation

55 LE 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

56 LE Prophase I of meiosis Tetrad Nonsister chromatids Chiasma, site of crossing over Recombinant chromosomes Metaphase I Metaphase II Daughter cells

57 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

58 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

59 Explain how junior and sis are not identical.

60 LE 8-17a Coat-color genes Eye-color genes Brown Meiosis White Tetrad in parent cell (homologous pair of duplicated chromosomes) Chromosomes of the four gametes Black C E e c C C c c E e E e Pink

61 LE 8-18b Coat-color genes Eye-color genes ( homologous pair of chromosomes in synapsis) Tetrad Breakage of homologous chromatids Joining of homologous chromatids Chiasma Separation of homologous chromosomes at anaphase  Separation of chromatids at anaphase  and completion of meiosis Parental type of chromosome Recombinant chromosome C C E E e e c c c c e E C CE e e E c C EC c e E C c e Parental type of chromosome Gametes of four genetic types

62 LE 8-18ba Coat-color genes Eye-color genes (homologous pair of chromosomes in synapsis) Tetrad Breakage of homologous chromatids EC c e E C c e Joining of homologous chromatids

63 LE 8-18bb Joining of homologous chromatids Chiasma Separation of homologous chromosomes at anaphase  Separation of chromatids at anaphase  and completion of meiosis E C c e E C c e e C c E

64 LE 8-18bc Separation of chromatids at anaphase  and completion of meiosis Parental type of chromosome Recombinant chromosome Parental type of chromosome Gametes of four genetic types E C c e C c E e

65 Table 8.22

66 Review of mitosis and meiosis

67 LE 8-un149 I NTERPHASE (cell growth and chromosome duplication) S (DNA synthesis) Cytokinesis Mitosis Genetically identical “daughter cells” M ITOTIC P HASE (M) G1G1 G2G2 (division of cytoplasm) (division of nucleus)

68 LE 8-un149s Haploid gametes (n  23) Egg cell Sperm cell n Fertilization Meiosis n Multicellular diploid adults Diploid zygote (2n  46) H UMAN LIFE CYCLE Mitosis and development 2n

69 LE 8-un1 Sister chromatids Homologous pair of chromosomes


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