Meiosis

• Where does meiosis take place?


  Reproductive organs - ovaries and teste where gametes are made
• In meiosis, what happens to the chromosome number?


  Halved
• How many daughter cells are produced in meiosis?


  Four genetically different daughter cells
• What happens before meiosis can occur?


  During interphase - each chromosome replicates (chromosomes now consist of two sister chromatids), DNA replicated semi conservatively,
• What are the two divisions in meiosis and what happens in them?


  Meiosis I: Homologous chromosomes separate. Meiosis II: Sister chromatids separate.
• What are homologous chromosomes?


  Homologous chromosomes are pairs of chromosomes that have the same genes in the same order, but may have different versions of those genes.
• What are the stages that take place in the meiotic division? 


  Prophase 1, metaphase 1, anaphase 1, telophase 1
• What must happen before the prophase 1 of meiosis?


  During interphase - DNA is replicated semi conservatively , chromosomes are replicated
• What happens in prophase one of meiosis?


  DNA condenses, nuclear membrane disappears, centrioles begin to move to the poles, homologous chromosomes pair up

  and line alongside each other, each pair of homologous chromosome is called by bivalent, crossing over occurs
• What is crossing over?


  It is a type of genetic variation, it happens in prophase one only, chromosomes tangle at points called chiasmata (some may break and rejoin), produces different arrangement alleles
• What happens in metaphase one?


  Nuclear membrane breaks down, spindle fibres form, bivalent moves to the equator and attach to the spindle fibres by the centromere
• What happens in anaphase one?


  Spindle fibres contract and pull chromosomes towards poles, homologous chromosome separate, bivalent begins to split
• What happens in telophase one?


  Nuclear membrane reforms around each set of chromosomes, spindle fibres disappear
• What happens after telophase one?


  Cell divides by cytokinesis
• What happens before prophase two?


  A short interphase takes place again, no DNA replicated in this interphase, the chromosomes uncoil
• what stages takes place second miotic division?


  prophase ll, metaphase ll, anaphase ll, telophase ll
• What happens in metaphase two?

  Spindle fibres form at right angles to the first, chromosomes move to the equator and spindle fibres attach to chromosomes by the centromere
• What happens Anaphase 2? 


  Centromere divides, sister chromatids move towards opposite poles
• What happens in Telophase two?


  Each chromatid is now called a chromosome, nuclear membrane reforms around each group of chromosomes, spindle fibres disappear and centrioles may divide
• what happens in prophase two?


  DNA condenses, nuclear membrane disappears, centrioles begin to move
• What happens after telophase 2?


  Cytokinesis - it forms four haploid daughter cells which are genetically different, division of cell membrane cytoplasm
• What are the ways genetic variation can be achieved in cell division


  crossing over, independent assortment, mutation
• What is independent assortment?

  • Due to random distribution and subsequent segregation of maternal and paternal chromosomes in the homologous pairs during anaphase one
  • It is random to which side of the pole of the cell the chromosomes are pulled to
• What is mutation in cell division?


  Genetic change During interphase when DNA is replicated