Meiosis & Variation

• Prophase I

  • Synapsis - pairing of homologous chromosomes
  • Crossing over between non sister chromatids results in new combinations of alleles from the homologous chromosomes
  • Chiasma - point of join between non sister chromatids
• Metaphase I

  • Spindle forms
  • Homologous pairs become attached to individual microtubules of the spindle by their centromeres
  • Homologous pairs are arranged at the equator of the spindle
• Anaphase I

  • Chromosomes of each homologous pair move to opposite poles of the spindle but the individual chromatids are still attached by their centromeres
• Telophase I

  • Homologous chromosomes arrived at opposite poles
  • Chromosomes tend to uncoil
  • Nuclear envelope reform around both nuclei
  • Spindle breaks down
  • Goes directly into Meiosis II
• Variation

  1. Independent assortment of maternal and paternal homologous chromosomes
  2. The way the chromosomes of each homologous pair line up at the equator of the spindle in meiosis I is entirely random
  3. Crossing over between maternal and paternal homologous chromosomes
  4. New combinations of alleles on the chromosomes of the haploid cells produced
  5. Random fusion of male and female gametes in sexual reproduction