Mitosis

For the OCR AS biology specification 

To remember stages of mitosis: 

 PMAT = Prophase Metaphase Anaphase Telophase

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Four stages of Mitosis - 1 & 2

Prophase - the chromosomes become visible under the microscope.

  • The chromosomes condense, getting shorter and fatter - each chromosome has two chromatids 
  • Centrioles move to opposite ends of the cell - forms a network of protein fibres (spindle)
  • Prophase ends at the point where the nuclear membrane surrounding the nucleus breaks down, releasing the chromosomes into the cytoplasm.

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Metaphase - the chromosomes in the middle of cell

  • Chromosomes come to the equator of the cell.
  • Centrioles organise microtubles into the spindle that stretches across cell.
  • The spindle attaches to each chromosome at its centromere and running to either pole of the cell.
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Four stages of Mitosis - 3 & 4

Anaphase - the chromosomes pull apart

  • Chromatids break apart at centromere 
  • the spindle fibres contract and pull the chromosomes  
  • One chromatid goes towards each pole of the cell - once separated the chromatids are chromosomes.

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Telophase - Two nuclei

  • Two new nuclear membranes form around the two groups of chromosomes.
  • The chromosomes uncoil and become long and thin again
  • Telophase finishes when the chromsomes fade and become invisible
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Interphase and Cytokinesis

Note: Interphase come before mitosis in the cell cycle

Interphase - Cells grow and replicate DNA ready for division. 

  • Chromosomes are invisible under light microscope
  • The cell carries out normal functions and prepares to divide.
  • DNA in cell is unravelled and replicates - doubles genetic content
  • Organelles are replicated, ATP content is increased - energy for cell divison 

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Note: Cytokinesis is after mitosis in the cell cycle

Cytokinesis - Divison of cytoplasm

  • Microtubles form a draw string just inside the membrane  
  • The membrane fuses as it is 'pinched in'. 
  • A new cell surface membrane is made on either side to enclose two seperate cells.
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Extra mitosis information

Mitosis is needed for:

  • Growth and Repair 
  • Replacement of cells
  • Asexual reproduction

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  • The number of chromosomes in a nucleus stays the same
  • The genetic information passed to daughter cells is identical
  • Two nuclei are formed and no genetic variation occurs

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  • Mitosis does not occur in prokaryotes
  • In plants only special cells called meristem are capable of mitosis and cytokinesis. 
  • Plants do not have centrioles - tubulin protein threads are made instead
  • In plants cytokinesis starts with formation of a cell plate - new cell membrane and cell wall are laid along it.
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