• Within multicellular organisms,?not all cells retain the ability to divide
• Eukaryotic cells that do retain the ability to divide show a cell cycle
• Mitosis?is part of a?precisely controlled process?known as the?cell cycle
• The cell cycle is the?regulated sequence of events?that occurs?between one cell division and the next
• The cell cycle has three phases:
  • interphase
  • nuclear division (mitosis)
  • cell division (cytokinesis)

   

• The length of the cell cycle is very variable depending on environmental conditions, the cell type and the organism
  • For example, onion root tip cells divide once every 20 hours (roughly) but human intestine epithelial cells divide once every 10 hours (roughly)

   

• The movement from one phase to another is triggered by chemical signals called?cyclins

The cell cycle. S = synthesis (of DNA); G = gap; M = mitosis

Interphase

• During Interphase the cell increases in mass and size and carries out its normal cellular functions (eg. synthesising proteins and replicating its DNA ready for mitosis)
• Interphase consists of?three?phases:
  • G₁?phase
  • S phase
  • G₂?phase

   

• It is at some point during the G₁?phase a?signal?is received telling the cell to?divide?again
• The?DNA in the nucleus replicates?(resulting in each chromosome consisting of two identical sister chromatids)
• This phase of the interphase stage of the cell cycle is called the?S phase?–?S?stands for?synthesis?(of DNA)
  • The S phase is relatively short

   

• The gap between the previous cell division and the S phase is called the?G₁?phase?–?G?stands for?gap
  • Cells make the RNA, enzymes and other proteins required for growth?during the G₁?phase

   

• Between the S phase and next cell division event the?G₂?phase?occurs
  • During the G₂?phase, the?cell continues to grow and the new DNA that has been synthesised is checked?and any errors are usually repaired
  • Other preparations for cell division are made (eg. production of tubulin protein, which is used to make microtubules for the mitotic spindle)

   

• Interphase = G₁?+ S + G₂

Nuclear division (mitosis)

• Follows interphase
• Referred to as the?M phase?–?M?stands for?mitosis
• Cell growth stops?during the M phase

Cytokinesis

• Follows M phase
• Once the nucleus has divided into two genetically identical nuclei, the?whole cell divides?and one nucleus moves into each cell to?create two genetically identical daughter cells
• In animal cells, cytokinesis involves?constriction of the cytoplasm?between the two nuclei and in plant cells a new cell wall is formed

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