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MEIOSIS

MEIOSIS. What is it? Making of gametes Sex Cells Sperm (pollen) in males Eggs (ova) in females Diploid Haploid Involves: Cell division 2 nuclear divisions Reduction of chromosome number by half. The human life cycle. Haploid gametes ( n = 23). Egg cell. Sperm cell. MEIOSIS.

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MEIOSIS

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  1. MEIOSIS • What is it? • Making of gametes • Sex Cells • Sperm (pollen) in males • Eggs (ova) in females • Diploid Haploid • Involves: • Cell division • 2 nuclear divisions • Reduction of chromosome number by half

  2. The human life cycle Haploid gametes (n = 23) Egg cell Sperm cell MEIOSIS FERTILIZATION Diploidzygote (2n = 46) Mitosis anddevelopment Multicellulardiploid adults (2n = 46)

  3. Homologous Chromosomes • Somatic cells of each species contain a specific number of chromosomes • Human cells have 46, making up 23 pairs of homologous chromosomes • In each homologous pair - • ~ 1 comes from “Mom” • ~ 1 comes from “Dad” Chromosomes Centromere Sister chromatids

  4. Interphase • Just like “Interphase” of Mitosis • G1 ~ ‘Gap’ 1 = growth and preparation • S ~ ‘Synthesis’ = DNA replication to make exact copies of the chromosomes • G2 ~ ‘Gap’ 2 = growth and shape change

  5. Meiosis I ~ Reduction Division • Prophase I • Chromatin thickens into chromosomes • You can now see them under a light microscope • Nuclear membrane begins to disintegrate • Centrioles migrate to opposite poles • Homologues do something unique to meiosis

  6. Prophase I - continued • Homologous Pairs line up next to each other • Synapsis • forms a tetrad • Crossing Over • pieces of the chromosomes ‘switch’ places • Stay connected at Chiasma(ta) • MOST IMPORTANT ~ for genetic diversity

  7. Metaphase I • Attached homologues line up on metaphase plate • Anaphase I • Tetrads separate at chiasma(ta) • ** sister chromatids are STILL together • Telophase I • Homologues move to poles • 2 nuclear membranes form (in some cells) ** Chromosome number is now HAPLOID ~there is now half the number of original chromosomes from the parent.

  8. Interkinesis • Stage between Meiosis I and II • Resting stage • NOT another Interphase • NO DNA replication • Strictly a “recovery” phase or rest period before moving on.

  9. Meiosis II – like Mitosis • Prophase II • Just like Meiosis I – but NO crossing over • Metaphase II • Sister chromatids line up on metaphase plate • Usually at right angles to meiosis I • Anaphase II • Sister chromatids separate at centromere and move to opposite poles • Telophase II and Cytokinesis • 4 new nuclei are formed • Each are haploid

  10. Crossing Over = Genetic Diversity

  11. Mitosis and Meiosis Compared

  12. WEB LINKS • http://www.lewport.wnyric.org/jwanamaker/animations/meiosis.html • http://www3.telus.net/byngscience/sc10/Sc10~Biology/s_tan_division.swf • http://www.pbs.org/wgbh/nova/baby/divi_flash.html

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