meiosis

• meiosis = sexual reproduction, produces gametes, gentetically different
• 1. prophase 1

  • homologous chromosomes pair up (bivalent)
  • bivalent condenses and the chromosomes join at several points (chiasma)
  • some genetic material is exchanged (crossing over)
  • centrioles start to form the spindle and the nuclear membrane begins to break down
• 2. metaphase 1:

  • bivalents arrange themselves along the equator
  • independent assortment of chromosomes creates variation
• 3. anaphase 1:

  • the centromeres do not divide - both chromatids are pulled towards opposite poles by the spindle fibres
  • chromosomes separated into 2 haploid sets of chromosomes in the daughter cells
  • independent assortment creates variation
• 4. telophase 1:

  • homologous chromosomes are now at opposite ends of the cell
  • chromosome number has been halved
  • each chromatid is genetically different
  • nuclear membrane begins to reform and cell splits
• 5. prophase 2:

  • nuclear membrane breaks down
  • chromatids shorten and thicken
  • centrioles move to opposite poles and form the spindle - often at right angle to the first ones
• 6. metaphase 2:

  • chromatids arrange themselves along the equator
• 7. anaphase 2:

  • centromeres divide and spindle fibres pull to opposite poles - chromatids now called chromosomes
  • independent assortment of the chromatids creates genetic variation
• 8. telophase 2:

  • chromosomes uncoil and lengthen
  • spindle disappears and nuclear membrane reforms
  • each nucleus now has half the number of chromosomes (haploid)
• spermatogenesis = making of sperm cells
• oogenesis = making of egg cells (ovum)
• gametogenesis = formation of gametes (sex cells)