Meiosis

• Meiosis- a reduction division resulting in 4 genetically different daughter cells which each contain half a set of genetic material (haploid)
• Crossing over

  During prophase 1 the homologous chromosomes pair to form bivalents

  Bivalents are 2 homologous chromosomes next to each other

  Crossing over genetic material can occur between the non-sister chromatids of bivalents

  Chiasmata formation when the chromatids cross over and parts of the chromatids (and their genetic material) are exchanged between the homologous chromosomes

  This results in new combination of alleles in the resulting gamete
• Independant assortment

  • During metaphase 1 the homologous pairs of chromosomes align at the equator of the cell.
  • It is random on which side of the equator the maternal and paternal chromosome could end up on either pole of the chromosome in the daughter cell.
  • As a result each gamete recieves different combination of maternal and paternal chromosomes
• Prophase1

  • DNA condenses and becomes visible as chromosomes
  • Homologous pairs of chromosomes joins up to form a bivalent
  • Centromeres can be seen and spindle fibres start to form
  • Crossing over may occur and chiasmata will form
  • Nuclear envelope breaks down
• Metaphase 1

  Bivalents line up along the equator of the spindle, they are attached by their centromeres

  Independent assortment occurs
• Anaphase 1

  The spindle fibres pull the bivalents apart by mircotubules

  Homologous pairs are seperated to opposite poles of the cell
• Telophase 1

  Chromosomes arrive at opposite poles of the cell

  Spindle fibres break down

  Nuclear envelope reforms around the 2 groups of chromosomes and nucleoli reform
• Prophase 2

  Nuclear envelope breaks down and chromosomes condense

  Spindle forms at a right angle to the old one
• Metaphase 2

  Sister chromatids line up single file along the equator of the spindle

  Chromatids are attached to the spindle by their centromere
• Anaphase 2


  Spindles seperate sister chromatids and are pulled to opposite poles of the cell
• Telephase 2

  Nuclear membranes form around each group of chromosomes and spindle fibres break down
• Cytokinesis

  Cytoplasm divides into 4 haploid daughter cells