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Chapter 12: Sexual Reproduction and Meiosis

Chapter Outline

Chapter 12: Sexual Reproduction and Meiosis

 

12.0 Introduction
  1. Most Cells Reproduce Sexually

    1. Gametes Are a Product of Special Division Called Meiosis fig 12.1

    2. Sexual Reproduction Generates Genetic Diversity

12.1 Meiosis produces sexual gametes

  1. Discovery of Reduction Division

    1. Different Chromosome Numbers Are Found in One Organism

      1. van Beneden examined cells in the roundworm Ascaris

        1. Gametes contained 2 chromosomes

        2. Somatic cells contained four chromosomes

    2. Fertilization

      1. Zygotes are produced by fusion of gametes

        1. Each gamete contains a single complement of genetic material

        2. The zygote contains two copies of each chromosome

      2. Fusion of gametes is called fertilization or syngamy

    3. Reduction Division

      1. Fusion of body cells would sequentially increase chromosome number

      2. Special reduction division serves to stabilize the chromosomes number

      3. Meiosis ensures constant chromosome number from one generation to next

  2. The Sexual Life Cycle

    1. Fertilization and Meiosis Constitute a Cycle of Sexual Reproduction fig 12.2

      1. Body cells of adult are diploid and possess two sets of chromosomes

      2. Gametes are haploid and possess a single set of genetic material

      3. An individual inherits genes from its father and its mother fig 12.3

        1. In humans, 23 chromosomes from the mother's egg

        2. In humans, 23 chromosomes from the father's sperm

    2. Somatic Tissues

      1. Life cycles show pattern of alternating chromosome numbers

      2. Alternate between diploid and haploid number fig 12.4 & 5

      3. After syngamy the zygote divides by mitosis

      4. All adult somatic cells are genetically identical to the zygote

      5. Unicellular organisms

        1. Individual cells function directly as gametes

        2. Zygote may divide mitotically or meiotically

    3. Germ-Line Tissues

      1. Cells that become meiotic cells are isolated early in development

      2. Called germ-line cells, are diploid like somatic cells

      3. Somatic cells undergo mitosis producing genetically identical diploid cells

      4. Germ-line cells undergo meiosis producing haploid gametes

12.2 Meiosis involves two nuclear divisions

  1. Unique Features of Meiosis

    1. Meiosis Consists of Two Rounds of Nuclear Division

    2. Comparing Mitosis and Meiosis

      1. Process has much in common with mitosis

      2. Meiosis has two unique features

        1. Synapsis

        2. Reduction Division

    3. Synapsis

      1. Homologous chromosomes (homologues) pair along their length fig 12.6a

      2. Crossing over: Genetic exchange occurs between pairs

      3. Chromosomes come together along equatorial plate

      4. Homologues drawn to opposite poles

      5. Clusters at poles are haploid

      6. Each chromosome still composed of two chromatids

    4. Reduction Division

      1. Chromosomes do not replicate between divisions

      2. After two divisions cell contains half the chromosomal complement fig 12.6b

      3. Second division is nearly identical to mitosis

      4. Sister chromatids dissimilar because of crossing over

      5. Process is continual, arbitrarily divided into stages

      6. Two stages called meiosis I and meiosis II

      7. Each stage divided into prophase, metaphase, anaphase and telophase fig 12.7

      8. Meiosis prophase I more complex than mitosis prophase

  2. Prophase I

    1. Preparation for Division

      1. Already replicated DNA coils even tighter

      2. Individual chromosomes become visible under light microscopy

      3. Chromosomes consist of two sister chromatids joined at centromeres

      4. Homologous chromosomes undergo synapsis, cross over segments and separate

    2. An Overview

      1. Divided into five sequential stages

      2. Leptotene: Chromosomes condense tightly

      3. Zygotene

        1. Homologues line up side by side in synapsis fig 12.8

        2. Forms resultant synaptonemal complex held together by protein latticework

      4. Pachytene

        1. Begins when synapsis is complete fig 12.9

        2. Each gene held in precise register with its corresponding gene

        3. DNA duplexes unwind

        4. Single strands of DNA pair with complementary strand from other homologue

      5. Diplotene

        1. Protein lattice disassembles

        2. Period of intense cell growth

        3. Chromosomes decondense, active transcription occurs

      6. Diakinesis

        1. Transition into metaphase

        2. Transcription stops, chromosomes recondense

    3. Synapsis

      1. Ends of chromatids attach to nuclear envelope at specific sites

        1. Membrane sites of homologues are adjacent

        2. Members of homologous pairs brought close together

      2. Homologues line up side-by-side

        1. Guided by heterochromatin sequences

        2. Process called synapsis

    4. Crossing Over

      1. Recombination nodules are very large protein assemblages

        1. Nodule spans central element of synaptonemal complex

        2. Act as large multienzyme recombination machines

      2. Complex series of events where DNA segments are exchanged between sister chromatids

      3. Two to three events per chromosome pair in humans

      4. When process is complete synaptonemal complex breaks down

      5. Homologous chromosomes released from the nuclear membrane

      6. Homologues do not separate completely

        1. Sister chromatids held together by their common centromere

        2. Paired homologues held together at points of crossing over

    5. Chiasma Formation

      1. Points of crossing over may be visible as X-shaped chiasma fig 12.10

        1. Chiasma indicates that two chromatids have exchanged parts fig 12.11

        2. Chiasma move to ends of arms as chromosomes separate

  3. Metaphase I

    1. Events of the Second Stage of Meiosis I

      1. Nuclear envelope disperses, microtubules form spindle as in mitosis

      2. Formation of terminal chiasmata

        1. Position of chiasma when reaches it ends of chromosome

        2. One side of centromere faces outward

        3. One side of centromere faces other homologue fig 12.12

        4. Spindle microtubules only attach to kinetochore proteins on outer face of centromere

        5. Centromere of each homologue attached to only one pole

        6. In mitosis kinetochores on both sides attach to microtubules

      3. Joined homologues line up on metaphase plate

      4. Attachment of homologue to a pole is random fig 12.13

      5. Alignment of chromosomes during metaphase I fig 12.14

  4. Completing Meiosis

    1. Anaphase I

      1. With completion of spindle attachment, microtubules shorten

      2. Chiasma broken, centromeres pulled toward each pole

        1. Chromosomes dragged along

        2. Individual centromeres not pulled apart

        3. Sister chromatids do not separate

      3. Each pole has a complete set of haploid chromosomes

        1. Each set contains one member of each homologous pair

        2. Poles receive homologues randomly

      4. Genes on different chromosomes assort independently

    2. Telophase I

      1. Each pole has full complement of chromosomes clustered at poles

      2. Nuclear membrane reforms around each new cluster

      3. Each chromosome exists as sister chromatids joined by centromere

      4. Chromatids are not identical because of crossing over fig 12.15

      5. Cytokinesis may or may not occur at this point

    3. The Second Meiotic Division

      1. Is a simple mitotic division using the products of meiosis I

      2. Prophase II: Nuclear envelope breaks down, new spindle forms

      3. Metaphase II: Spindle apparatus binds to sides of centromeres

      4. Anaphase II

        1. Spindle fibers contract

        2. Centromeres divide

        3. Sister chromatids drawn to opposite poles

      5. Telophase II: Nuclear envelopes reform

    4. Completion of the Process

      1. End result is four haploid complements of chromosomes

      2. In animals, cells develop into gametes

      3. In plants, fungi, protists cells may proliferate via mitotic divisions

12.3 The evolution of sex led to increased genetic variability

  1. Why Sex?

    1. Not All Reproduction Is Sexual

      1. Asexual reproduction

        1. Individual inherits all chromosomes from one parent

        2. Individual is genetically identical to parent

      2. Bacterial cells reproduce by binary fission

      3. Protists divide asexually unless under stress

      4. Plants and multicellular organisms frequently reproduce asexually

      5. Animals may reproduce by budding off localized masses of cells

      6. Development from an unfertilized egg via parthenogenesis fig 12.16

        1. Example: Bees

          1. Fertilized eggs become diploid females

          2. Unfertilized eggs become haploid males

        2. Examples: Lizards, fish, amphibians

    2. Recombination Can Be Destructive

      1. Problems associated with sexual reproduction

        1. Advantage to species which benefit from genetic variability

        2. Evolution occurs because of changes at level of the individual

      2. Recombination is evolutionarily both constructive and destructive

        1. Segregation of chromosomes disrupts beneficial gene combinations

        2. Diverse progeny will be less well-adapted than parents

        3. Complex adaptations are less likely to benefit from recombination

    3. Synapsis Evolved to Repair DNA

      1. What are benefits to sexual reproduction?

        1. Meiotic recombination among protists is often absent

        2. Sex may only occur under stressful conditions

        3. In some protists diploid is transient or only haploid phase exists

          1. With stress haploids fuse forming diploid zygote

          2. Resulting diploid may not persist

      2. Sex may have evolved in protists to repair DNA damage

        1. Particularly double-stranded breaks in DNA

        2. Breaks induced by radiation or chemicals

        3. Repair of such damage is necessary in longer-lived organisms

        4. DNA repair through mechanism of synaptonemal complex

        5. Transient diploid stage allows for such repair

        6. Special yeast mutations

          1. Repair system inactivated for double-strand breaks

          2. Crossing over also prevented

  2. The Evolutionary Consequences of Sex

    1. Principal Factors in the Evolution of the Eukaryotes

      1. Independent assortment fig 12.17

        1. Reassortment of genetic material occurs during meiosis

        2. Represents an enormous factor in initiation of genetic variability

          1. In humans 23 chromosomes are from each parent

          2. Each chromosome segregates independently of all others

          3. Gamete possibilities equals 223 (over eight million)

      2. Crossing over

        1. Exchange of DNA between sister chromatids further adds to the variability

        2. Number of possible recombinations virtually unlimited

      3. Random fertilization

        1. Each new zygote combination produced independently

        2. Fertilization squares the number of possibilities (70 trillion)

    2. Importance of Generating Diversity

      1. Evolutionary consequences of sex are profound

        1. Genetic diversity is raw material of evolution

        2. Pace of evolution increased with greater genetic diversity

        3. Example: Thoroughbred race horses

          1. All descended from small number of individuals, limited genetic variability

          2. Winning times in races not improved in recent history

      2. Evolutionary process is revolutionary and conservative

        1. Revolutionary as the pace is quickened by genetic variability

        2. Conservative as variation is not always favored by selection

        3. In vertebrates, the evolutionary premium is on versatility, thus sex

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