Meiosis

• Meiosis

  The nuclear division that gives rise to haploid gametes
• interphase

  the DNA content of the cell doubles as DNA replicates

  G1- cell increases in size and mass, more organelles

  S- dna replicated

  G2- checks for errors
• Prophase 1

  -at the start of prophase the chromosomes are indistinct in the nucleus

  -the centrioles replicate and migrate to the poles of the cells

  -chromosomes become visible as they wind around histone proteins

  -nuclear membrane disappears

  -homologous chromosomes pair up to form bivalents

  -the arms of chromisomers may cross over this is called chiasmata

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• metaphase 1

  -the spindle fibres form- these are micro tubules organised by the centrioles

  -the bivalents attach to the spindle fibres at the Equator of the cell
• Anaphase 1

  -the spindles retract, pulling the homologous chromomes of the bivalent apart
• telophase 1

  -having reached the poles of the cell the chromosomes unwind and become indistinct

  -nuclear membrane reforms

  -cytokinesis occurs
• prophase 2

  -centrioles replicate and migrate to the poles of the cell

  -chromosomes become visible by supercoiling
• metaphase 2

  -the spindle fibres from- these are microtubules organised by the centrioles

  -the chromosomes attach to the spindle fibres at the equator of the cell
• anaphase 2

  -the spindles retract pulling the chromosomes to the poles
• telophase 2

  reached the poles of the cells and the chromosomes unwind an become indistinct

  nuclear membrane reforms

  cytokinesis occurs
• sources of variation 1

  crossing over during prophase 1

  • there us recombination of genetic material
• sources of variation 2

  random assortment of chromosomes during metaphase 1

  -chromosomes orientate themselves independently of the other homologous pair