Meiosis and Reproduction PDF

Sex and meiosis Modes of reproduction Diploid/haploid and meiosis Meiosis I and Meiosis II Key differences Why sex?  First, some terms: Chromosome Chromatid Homologous chromosomes Sister Chromatids Haploid Diploid  First, some terms: Chromosome Gene Locus Allele G...

Sex and meiosis Modes of reproduction Diploid/haploid and meiosis Meiosis I and Meiosis II Key differences Why sex?  First, some terms: Chromosome Chromatid Homologous chromosomes Sister Chromatids Haploid Diploid  First, some terms: Chromosome Gene Locus Allele Gamete Somatic Gametic Sex and meiosis Modes of reproduction Diploid/haploid and meiosis Meiosis I and Meiosis II Key differences Why sex?  Asexual vs Sexual Asexual reproduction  Asexual vs Sexual Asexual reproduction Mitosis Clones  Asexual vs Sexual Asexual reproduction Mitosis Clones Sexual reproduction Meiosis Fertilization Genetic variation  Asexual vs Sexual Sexual reproduction Meiosis Fertilization Genetic variation Gametes fuse to make zygote  Asexual vs Sexual Sexual reproduction Meiosis Fertilization Genetic variation Gametes fuse to make zygote Must halve chromosome number Meiosis  Asexual vs Sexual Sexual reproduction Meiosis Fertilization Genetic variation Gametes fuse to make zygote The Variety of Sexual Life Cycles The alternation of meiosis and fertilization is common to all organisms that reproduce sexually Sexual life cycles differ in the timing of meiosis and fertilization Sex and meiosis Modes of reproduction Diploid/haploid and meiosis Meiosis I and Meiosis II Key differences Why sex? Haploid vs Diploid  Haploid vs Diploid Key Maternal set of 2n = 6 chromosomes (n = 3) Paternal set of chromosomes (n = 3) Sister chromatids of one duplicated chromosome Centromere Two nonsister Pair of homologous chromatids in chromosomes a homologous pair (one from each set)  Haploid vs Diploid Diploid cells give rise to haploid cells via meiosis Meiosis reduces chromosome number, producing up to 4 haploid cells from one diploid cell Meiosis has two successive cell divisions after only one DNA replication Meiosis I Meiosis II Sex and meiosis Modes of reproduction Diploid/haploid and meiosis Meiosis I and Meiosis II Key differences Why sex? Figure 13.7-3 Interphase Pair of homologous chromosomes in diploid parent cell Duplicated pair Chromosomes of homologous duplicate chromosomes Sister Diploid cell with chromatids duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unduplicated chromosomes  Meiosis: 2 divisions MEIOSIS I: Separates homologous chromosomes MEIOSIS I: Separates sister chromatids Telophase I and Telophase II and Prophase I Metaphase I Anaphase I Prophase II Metaphase II Anaphase II Cytokinesis Cytokinesis Centrosome Sister chromatids (with centriole pair) remain attached Sister Chiasmata Centromere chromatids (with kinetochore) Spindle Metaphase plate During another round of cell division, the sister chromatids finally separate; Cleavage four haploid daughter cells result, containing unduplicated chromosomes. furrow Homologous Sister chromatids Haploid daughter Homologous Fragments chromosomes separate cells forming chromosomes of nuclear separate envelope Microtubule Each pair of homologous Two haploid cells attached to chromosomes separates. form; each chromosome kinetochore still consists of two sister chromatids. Duplicated homologous Chromosomes line up chromosomes (red and blue) by homologous pairs. pair and exchange segments; 2n = 6 in this example.  Meiosis I. Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Centrosome (with centriole pair) Sister chromatids remain attached Sister Chiasmata Centromere chromatids Spindle (with kinetochore) Metaphase plate Cleavage furrow Homologous Homologous Fragments chromosomes chromosomes of nuclear separate envelope Microtubule Each pair of homologous Two haploid attached to chromosomes separates. cells form; each kinetochore chromosome Chromosomes line up still consists Duplicated homologous of two sister by homologous pairs. chromatids. chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example.  Meiosis I: Prophase I Chromatin condenses to form chromosomes Homologous chromosomes pair - Synapsis tetrad – the resulting structure, with 4 total chromatids tetrad held together by synaptonemal complex  Meiosis I: Prophase I Chromatin condenses to form chromosomes Homologous chromosomes pair – Synapsis tetrad – the resulting structure, with 4 total chromatids tetrad held together by synaptonemal complex Crossing over- between non-sister chromatids Meiosis I: Prophase I  Meiosis I: Prophase I Terms: homologous chromosomes Sister chromatids Non-sister chromatids Synapsis Tetrad Crossing over Recombination Chiasmata  Meiosis I: Prophase I By the end of prophase I: spindle has formed nuclear membrane has dissolved homologous chromosomes attached by kinetochores to spindle fibers from opposite poles homologous chromosomes held together at chiasmata  Meiosis I: Metaphase I Tetrads arranged at metaphase plate Each chromosome of tetrad attached to opposite pole Chromosome line up independently of each other  Meiosis I: Metaphase I Chromosome line up independently of each other  Meiosis I: Anaphase I Homologous chromosomes separate Move to opposite poles “Maternal” or “paternal” chromosome sets are mixed and distributed randomly  Meiosis I: Telophase I Generally: Spindle fibers disintegrate Chromosomes partially decondense Nuclear membranes may form around the genetic material Cytokinesis occurs Meiosis I: Interkinesis  Meiosis I: Interkinesis Period between meiosis I and meiosis II varies in length and distinctiveness interkinesis differs from interphase because there is no S phase (no DNA replication) typically, interkinesis is brief (some cells skip it altogether) Now have 2 haploid cells Meiosis II: Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Sister chromatids Haploid daughter separate cells forming  Meiosis II: Prophase II Similar to prophase of mitosis Usually very short  Meiosis II: Metaphase II Similar to metaphase of mitosis Single chromosomes align at metaphase plate Sister chromatids connected to spindle fibers from opposite poles  Meiosis II: Anaphase II Sister chromatids separate  Meiosis II: Telophase II Similar to mitotic telophase: Spindle disintegrates Chromosomes decondense Nuclear membranes reform Cytokinesis begins during telophase II and ends shortly thereafter Sex and meiosis Modes of reproduction Diploid/haploid and meiosis Meiosis I and Meiosis II Key differences Why sex? Sex and meiosis Modes of reproduction Diploid/haploid and meiosis Meiosis I and Meiosis II Key differences Why sex?  Sex produces genetic variation Mutations (changes in an organismʼs 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  Sex produces genetic variation Behavior of chromosomes during meiosis and fertilization responsible for most variation Three mechanisms contribute to genetic variation 1. Crossing over 2. Independent assortment of chromosomes 3. Random fertilization  Crossing Over = Recombination Crossing over produces recombinant chromosomes Combines DNA from two parents into a single chromosome  Independent Assortment Homologous pairs of chromosomes orient randomly at metaphase I of meiosis Each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of the other pairs  Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4  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  Why sex? The origin and maintenance of sex is an evolutionary puzzle Sexual reproduction dilutes the genes from the “best adapted” individuals, and thus must offer a significant advantage or asexual reproduction will win out Even when sex clearly benefits a population or species, it must directly benefit individuals or it will lose out via evolution