meiosis

• Meiosis


  Produces 4 haploid daughter cells that are genetically different from each other

  • Involve two nuclear divisions
  • From single diploid parent cell
• Variation

  • genetically different daughter cells result from the independent segregation of homologous chromosomes
  • crossing over between homologous chromosomes results in further genetic variation among daughter cells.
• Number of possible chromosome combinations in daughter cells can be calculated using 2^n, where n is the number of homologous pairs
• Random fertilisation


  • Egg and sperm fuse randomly
  • 2^n possible chromosome combinations in gametes
  • 2^(n/2) possible chromosome combinations when considering random fertilisation
• Crossing over


  • Different combination of alleles
  • When homologous chromosomes line up, parts of chromatids can twist around each other
  • Tension put on chromatids, pairs of chromatid break
  • Broken parts recombine with another chromatid - new allele combinations made
• Meiosis


  • Two nuclear divisions
  • Haploid cells (one set of chromosomes)
  • Introduces genetic variation
• Mitosis


  • One nuclear division
  • Diploid cells (two sets of chromosomes)
  • Creates genetically identical cells
• Independent segregation


  Different combination of chromosomes

  • When lining up at the equator in metaphase, the side to which each sister chromatid (of the homologous pair) ends up on is random
• Meiosis I


  • Prophase I - chromosomes condense (become visible), nuclear membrane breaks down
  • Metaphase I - homologous pairs line up next to each other, spindle fibers form & attach to centromeres
  • Anaphase I - homologous pairs pulled to opposite poles
  • Telophase I - nuclei reform, cell divides
• Meiosis II


  • Prophase II - nuclei break down
  • Metaphase II - chromosomes line up at equator, spindle forms & attaches to centromeres
  • Anaphase II - chromatids pulled apart to opposite poles
  • Telophase II - nuclei reform, cell divides