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Meiosis

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Meiosis

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  1. Meiosis • Summary of process: • Process to reduce number of chromosomes in sex cells (diploid germ-line cells produces haploid gametes) • Meiosis: • Two nuclear divisions: reduction division (Meiosis I) and mitotic-like division (Meiosis II) • Produces 4 genetically different haploid gametes • Part of sexual reproduction How are new organisms produced?

  2. From Mom From Dad Meiosis • Unique features of meiosis: • Synapsis: • Meiosis: • Prophase I • Homologous chromosomes (= ‘matching’ chromosomes) pair together lengthwise How are new organisms produced?

  3. synapsis Meiosis • Unique features of meiosis: • Crossingover: • Meiosis: • Prophase I • Homologous chromosomes ‘exchange’ sections of their chromosomes • Location of crossing over = chiasma (plural, chiasmata) How are new organisms produced?

  4. Meiosis • Unique features of meiosis: • Reductiondivision: • Meiosis: • Anaphase I • Homologous chromosomes separate and move into different cells; no further chromosome replication occurs • Results in ‘daughter cells’ with half the number of chromosomes (one chromosome of each homologous pair) How are new organisms produced?

  5. Meiosis • Interphase I • Chromosomes (chromatin) replicate forming sister chromatids as in mitosis • Stages of meiosis: • Chromosomes begin to condense • Centrioles replicate (in animal cells) • All activity occurs within nucleus of germ-line cell How are new organisms produced?

  6. Meiosis • Prophase I: • Chromosomes condense • Stages of meiosis: • Centrioles move apart; spindle apparatus forms • Nuclear membrane breaks down • Synapsis (homologous chromosomes pair) • Crossing over between homologous chromosomes How are new organisms produced?

  7. Meiosis • Metaphase I: • Homologous pairs randomly line up at metaphase plate (chromosome combination in gametes depends on line up in metaphase -- reason for independent assortment) • Stages of meiosis: • Kinetochore microtubules attach to one side of each homologous chromosome How are new organisms produced?

  8. Meiosis • Anaphase I: • Kinetochore microtubules shorten; homologous chromosomes pulled to opposite poles (sister chromatids NOT pulled apart) • Stages of meiosis: • Each pole gets a haploid set of chromosomes (one or the other homologue) How are new organisms produced?

  9. Meiosis • Telophase I: • Nuclear membrane reforms around each haploid set of chromosomes • Stages of meiosis: • Cytokinesis usually occurs How are new organisms produced?

  10. Meiosis • Meiosis II: • Identical to mitosis except no chromosome replication during brief (if any) interphase II • Stages of meiosis: How are new organisms produced?

  11. Meiosis • Genetic recombination results from: • Arrangement of homologous chromosomes during metaphase I • Results of sexual reproduction: • Crossing over • Random fertilization • Genetic recombination produces genetic variation among individuals • Genetic variation necessary for adaptations by natural selection • Other source of genetic variation is mutation How are new organisms produced?