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MEIOSIS

Explore Meiosis, the process of creating gametes (sperm and eggs) involving nuclear divisions and chromosome reduction for genetic diversity and the human life cycle. Comparison with Mitosis and detailed phases explained.

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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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