meiosis

• • Homologous chromosomes are matching sets for chromosomes (they contain the same genes in the same loci)
  • Diploid cells contain homologous chromosomes
  • Haploid cells do not
• Meiosis is split into two phases

  1. Meiosis I
  2. Meiosis II

  Interphase happens before the first stages but not between the phases meaning we end up with half the chromosomes in each cell.
• Prophase 1 -

  Like in mitosis

  • Chromosomes condense
  • Nuclear envelope disintegrates
  • Nucleolus disappears
  • Spindle fibres begin to form

  But also…

  • Homologous chromosome pair up into bivalents
  • Crossing over starts to occur

  
• Metaphase 1 -

  Like in mitosis

  • Chromosomes line up in the middle/equator

  But also…

  • Homologous pairs line up together
  • Which chromosomes are facing each other in each pair however is random (this is called independent assortment)
• anaphase 1 -

  Like in mitosis

  • Chromosomes move to opposite poles

  However

  • Instead of chromatids moving,  the chromosomes move together to either poles
  • The ones from mother one way and father the other (depending on how they lined up)
• telophase 1 -

  Like in mitosis

  • Chromosomes assemble
  • The nuclear membrane reforms
  • Chromosomes unravel
• cytokinesis -

  Like in mitosis

  • Cells divide into two cells

  However

  • The number of chromosomes is reduced from 46 to 23.
  • BUT…These two haploid cells have double stranded chromosomes (two chromatids)
• prophase II

  Like in mitosis

  • Chromosomes condense
  • Nuclear envelope disintegrates
  • Spindle fibres begin to form

  However

  • This time we start with only 23 chromosomes
• metaphase II -

  Like in mitosis

  • Chromosomes line up in the middle

  This time

  • Individual chromosomes line up by themselvesindependent assortment again (this will make sense later)
  • There is also some
• anaphase II -

  Like in mitosis

  • Chromatids move to move to either pole
  • After centromeres divide
• telophase II -

  Like in mitosis

  • Chromosomes assemble at poles 
  • The nuclear membrane reforms
  • Chromosomes uncoil
  • Nucleolus becomes visible
• cytokinesis II -

  Like in mitosis

  • Cells divide into two cells

  However

  • Cells are genetically different
  • Cells are haploid
• Reason 1 – Independent assortment

  • In Meiosis I the chromosomes from your mum and dad line up randomly as so might go either way in any combination. This can result in any alleles facing either pole.
  • In Meiosis II crossing over means different genes on either chromatid can lead to more independent assortment
• Reason 2 – Crossing over