meiosis

• takes place in the reproductive organs prior to sexual reproduction - it results in 4 genetically distinct haploid cells
• in meiosis the diploid number of chromosomes is halved to haploid
• interphase occurs before meiosis 1 but not meiosis 2
• prophase 1

  1. homologous pairs form - each pair is bivalent
  2. chromosomes become visible because they condense - coil up become shorter and thicker
  3. centrioles separate and move to the poles of the cell
  4. spindle forms
  5. nuclear envelope disintegrates and nucleolus disappears
  6. genetic crossing over
• genetic crossing over

  at the chiasma a segment of DNA from one chromatid may be exchanged with the equivalent part from a chromatid of the homologous chromosome - source of genetic variation - occurs in prophase 1
• metaphase 1

  1. pairs of homologous chromosomes line up on the equator
  2. independent assortment
• independent assortment - homologous chromosome arrange themselves randomly at the equator - source of genetic variation
• anaphase 1

  1. chromosomes in bivalent pairs separate
  2. spindle shortens and pulls one from each pair to opposite poles
• telophase 1

  in some species the nuclear envelope reforms and the chromosomes decondense
• cytokinesis after telophase 1 - division of cytoplasm into 2 haploid cells
• prophase 2

  1. centrioles separate and organise a new spindle at a right angle to the old spindle
• metaphase 2

  1. chromosomes line up on the equator with chromosome attached to a spindle fibre by its centromere
  2. independent assortment
• anaphase 2

  1. centromeres divide and spindle fibres shorten pulling chromatids to opposite poles
• telophase 2

  1. at the poles the chromatids lengthen and can no longer be distinguished
  2. the spindle disintegrates and nuclear envelope and nucleoli reform
  3. cytokinesis occurs producing 4 daughter cells