The Cell Cycle

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Mitosis - Prophase

  • Chromosomes become visible as long, thin threads


  • Chromosomes start to coil up and become shorter


  • Centrioles divide and move to opposite poles of the nucleus


  • Spindle fibres form - extend from pole to pole


  • The nucleolus disappers and the nuclear envolope breaks down
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Mitosis - Metaphase

  • Chromosomes move towards the equator of the spindle


  • Attach themselves to the spindle by the centromere
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Mitosis - Anaphase

  • The centromeres holding each pair of chromatids together divide


  • The free chromatids move towards the poles - centromere first


  • This movement results from the contraction of the spindle fibres


  • As they shorten, they pull the chromatids apart
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Mitosis - Telaphase

  • The chromatids have reached the poles and can be regarded as distinct chromosomes


  • The nuclear envelope forms around each group of chromosomes


  • The nucleolus reappears


  • The chromosomes uncoil to form diffuse chromatin


  • The cytoplasm divides by cytokinesis
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  • Starts during telaphase


  • Centre of the cell pinches in - DIVISON FURROW


  • Forms due to the contraction of a ring made up of actin and myosin


  • The divison furrow deepens and the CSM on each side join up and two seperate cells result
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Meiosis - Prophase I

  • Chromosomes condense, thicken and coil
  • The paternal and maternal chromosomes come together in homologous pairs
  • The pairing of chromosomes is called SYNAPSIS
  • Each homologous pair of chromosomes is called a BIVALENT
  • As chromosomes pair, they shorten and twist around eachother, causing tension
  • Sections of chromatid may break of and exchange with corresponding sections of a different chromatid
  • The points they exchange are called CHIASMATA
  • This is called CROSSING OVER and leads to genetic recombination
  • The nucleoulus disappears and nuclear envelope breaks down
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Meiosis - Metaphase I

  • The spindle would have formed


  • Chromosomes assemble on the equator of the spindle

Differs from Metaphase in Mitosis as the chromosomes are joined in homologous pairs

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Meiosis - Anaphase I

  • The chromosomes in each bivalent seperate


  • Each pole receives only one of each homologous pair of chromosomes


  • The contraction of spindle fibres pulls the homologous chromosomes apart


  • Each pole receives a haploid number of chromosomes
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Meiosis Telophase

  • The chromosomes reach the opposite poles of the spindle


  • The nuclear envelope forms around each group of haploid chromosomes


  • Chromosomes stay in their condensed form so Meiosis II can take place


  • Cytokinesis occurs to produce two haploid cells
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Meiosis - Prophase II

  • New spindle fibres form at right angles to the old spindle
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Meiosis - Metaphase II

  • The seperate chromosomes arrange themselves on the equator of the spindle


  • Each chromosome attaches to the fibres by their centromere
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Meiosis - Anaphase II

  • The centromeres divide


  • The spindle fibres contract to pull the two chromatids to the poles, centromere first
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Meiosis - Telophase II

  • On reaching the poles, the chromatids lengthen and become indistinct


  • The spindle disappears and the nuclear envelope reforms


  • Cytokinesis takes place, forming four haploid cells that are all genetically different
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