Chapter Outline
INTRODUCTION TO MEIOSIS Different Chromosome Numbers Are Found in One Organism Gametes contain half as many chromosomes as somatic cells Zygotes are produced by fusion of gametes Each gamete contains a single complement of genetic material The zygote contains two copies of each chromosome Fusion of gametes is called fertilization or syngamy Fusion of body cells would sequentially increase chromosome number Meiosis Is an Important Event fig 12.1 Serves to stabilize the chromosomes number Is a reduction division process THE SEXUAL LIFE CYCLE Fertilization and Meiosis Constitute a Cycle of Sexual Reproduction fig 12.2 Body cells of adult are diploid and possess two sets of chromosomes Gametes are haploid and possess a single set of genetic material An individual inherits genes from its father and its mother fig 12.3 In humans, 23 chromosomes from the mother's egg In humans, 23 chromosomes from the father's sperm Life Cycles Show Pattern of Alternating Chromosome Numbers Alternate between diploid and haploid number fig 12.4 After syngamy the zygote divides by mitosis All adult somatic cells are genetically identical to the zygote Sexual Reproduction Varies by Kingdom Unicellular organisms Individual cells function directly as gametes Zygote may divide mitotically or meiotically Plants Haploid cells are produced through meiosis Cells divide mitotically to form multicellular haploid phase Special haploid cells differentiate into eggs or sperm Animals Gamete-producing cells differentiate from somatic cells early Referred to as germ line cells Somatic cells are diploid and reproduce by mitosis Diploid gamete-producing cells, produce haploid gametes by meiosis THE STAGES OF MEIOSIS Meiosis Consists of Two Rounds of Nuclear Division Process has much in common with mitosis Different due to two unique features Homologous chromosomes (homologues) pair along their length Crossing over: genetic exchange occurs between pairs Chromosomes come together along equatorial plate Homologues drawn to opposite poles Clusters at poles are haploid Each chromosome still composed of two chromatids Chromosomes do not replicate between divisions Second division is nearly identical to mitosis Sister chromatids dissimilar because of crossing over Process is continual, arbitrarily divided into stages Two stages called meiosis I and meiosis II Each stage divided into prophase, metaphase, anaphase and telophase fig 12.5 Meiosis prophase I more complex than mitosis prophase The First Meiotic Division Prophase I Individual chromosomes become visible under light microscopy Each chromosome replicated thus present as two sister chromatids Ends of chromatids attach to nuclear envelope at specific sites Membrane sites of homologues are adjacent Members of homologous pairs brought close together Homologues line up side by side in synapsis fig 12.6 Forms resultant synaptonemal complex Held together by protein latticework Each gene held in precise register with its corresponding gene DNA duplexes unwind Single strands of DNA pair with complementary strand from other homologue Synapsis initiates a complex event called crossing over DNA segments are exchanged between sister chromatids When process is complete synaptonemal complex breaks down Homologous chromosomes released from the nuclear membrane Homologues do not separate completely Sister chromatids held together by their common centromere Paired homologues held together at points of crossing over Points of crossing over may be visible as X-shaped chiasma fig 12.7 Chiasma indicates that two chromatids have exchanged parts Chiasma move to ends of arms as chromosomes separate Metaphase I Nuclear envelope disperses, microtubules form spindle as in mitosis Different from mitosis as homologues are paired Terminal chiasmata: when chiasma reaches ends of chromosome One side of centromere faces outward One side of centromere faces other homologue fig 12.8 Spindle microtubules can only attach to outer face of centromere Centromere of each homologue attached to only one pole Joined homologues line up on metaphase plate Attachment of homologue to a pole is random fig 12.9 Anaphase I With completion of spindle attachment, microtubules shorten Chiasma broken, centromeres pulled toward each pole Chromosomes dragged along Individual centromeres not pulled apart Sister chromatids do not separate Each pole has a complete set of haploid chromosomes Each set contains one member of each homologous pair Poles receive homologues randomly Genes on different chromosomes assort independently Telophase I Each pole has full complement of chromosomes Each chromosome exists as sister chromatids joined by centromere Chromatids are not identical because of crossing over Cytokinesis may or may not occur at this point The Second Meiotic Division fig 12.10 Is a simple mitotic division using the products of meiosis I Two clusters at either pole divide mitotically Spindle apparatus binds to sides of centromeres Centromeres divide Chromatids drawn to opposite poles End result is four haploid complements of chromosomes Nuclear envelopes reform In animals, cells develop into gametes In plants, fungi, protists cells may proliferate via mitotic divisions WHY SEX? Not All Reproduction Is Sexual Asexual reproduction Individual inherits all chromosomes from one parent Individual is genetically identical to parent Bacterial cells reproduce by binary fission Protists divide asexually unless under stress Multicellular organisms May reproduce by budding off localized masses of cells Sponges reproduce asexually by fragmentation Development from an unfertilized egg via parthenogenesis Example: bees Fertilized eggs become diploid females Unfertilized eggs become haploid males Example: vertebrates fig 12.11 Evolutionary Rationale for Sexual Reproduction Problems associated with sexual reproduction Advantage to species which benefit from genetic variability Evolution occurs because of changes at level of the individual Recombination is evolutionarily both constructive and destructive Segregation of chromosomes disrupts beneficial gene combinations Diverse progeny will be less well-adapted than parents Complex adaptations are less likely to benefit from recombination Trends for asexually-reproducing organisms Live in harsh habitats Premium on well-adapted, genetically uniform individuals Benefit to sexual reproduction is yet unknown Meiotic recombination among protists is often absent Sex may only occur under stressful conditions In some protists diploid is transient or only haploid phase exists With stress haploids fuse forming diploid zygote Resulting diploid may not persist Sex may have evolved in protists to repair DNA damage Particularly double-stranded breaks in DNA Breaks induced by radiation or chemicals Repair of such damage is necessary in longer-lived organisms DNA repair through mechanism of synaptonemal complex Transient diploid stage allows for such repair Special yeast mutations Repair system inactivated for double-strand breaks Crossing over also prevented THE EVOLUTIONARY CONSEQUENCES OF SEX Principal Factors in the Evolution of the Eukaryotes Reassortment of genetic material occurs during meiosis Represents an enormous factor in initiation of genetic variability In humans 23 chromosomes are from each parent Each chromosome segregates independently of all others Gamete possibilities equals 223 (over eight million) Fertilization squares the number of possibilities (70 trillion) Crossing over further adds to the variability Evolutionary Consequences of Sex Are Profound Genetic diversity is raw material of evolution Pace of evolution increased with greater genetic diversity Evolutionary Process Is Revolutionary and Conservative Revolutionary as the pace is quickened by genetic variability Conservative as variation is not always favored by selection Acts to preserve existing combinations of genes Greater in asexual organisms that are not highly mobile Live in extremely demanding habitats In vertebrates, the evolutionary premium is on versatility, thus sex