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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Cytokinesis

  • 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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