meiosis

• What is the purpose of meiosis?


  To form haploid gametes from diploid cells
• What occurs during Interphase one of meiosis?


  • Each chromosome is converted into two identical chromatids
  • Chromosomes duplicate
  • Centrosomes duplicate
• What happens to chromosomes during prophase one of meiosis?


  Chromosomes condense and become visible as sister chromatids
• What are the key events during prophase one (prometaphase) of meiosis?


  • Nuclear membrane breaks down
  • Spindle begins to form
  • Homologous chromosomes pair up forming a bivalent
  • Crossing over of genetic material occurs
• What is crossing over in meiosis?


  It is the exchange of genetic material between non-sister chromatids of homologous chromosomes
• What are the crossing points called where crossing over occurs?


  Chiasmata
• What happens during meiosis one after crossing over?


  • Formation of recombinant chromosomes
  • Homologous chromosomes align at the equator
  • Nuclear membranes restore around each group of chromosomes
• What is the outcome of meiosis one?


  Two separate daughter cells are formed
• How does variation occur during meiosis?


  • Crossing over creates recombinant chromosomes
  • Independent assortment of chromosomes
  • Random fertilization of eggs by sperm
• What is the significance of independent assortment in meiosis?


  It results in a mixture of paternal and maternal chromosomes in new cells
• How does random fertilization contribute to genetic variation?


  Any of millions of different sperm cells can fertilize one egg
• What is meiosis


  A reduction division to form haploid gametes from diploid cells
• What happens during meiosis 1 - interphase 1

  each chromosome is converted into two identical chromosomes. Chromosomes duplicate + centrosomes duplicate
• What happens in meiosis 1 - prophase 1


  Chromosomes condense and become visible - 2 sister chromatids
• What happens in meiosis 1 - prometaphase 1


  Nuclear membranes begin to from

  Homologous chromosomes pair up forming a bivalent

  RECOMBINATION / CROSSING OVER
• What is crossing over / recombination 


  Crossing over of non sister chromatids of homologous chromosomes pair + exchange of genetic material + the point where they cross = chiasmata

  Forming recombinant chromosomes
• What happens during independent assortment

  paternal and maternal chromosomes of homologous pair are randomly distributed along the equator
• What happens during meiosis 1 - metaphase 1 


  Homologous chromosomes align as pairs

  Independent assortment of chromosomes
• What happens during meiosis 1 - anaphase 1


  The spindle fibres contract , pulling homologous chromosomes to opposite poles
• What happens during meiosis 1 - telophase 1


  Chromosomes reach opposite poles of the cell

  Nuclear membranes reform
• What happens during meiosis 1 - cytokenisis 1


  Cleavage furrows forming two separate daughter cells
• What happens during meiosis two - prophase two

  Chromosomes are already condensed

  Nuclear membranes break down

  Spindles begin to form