• 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 = growth; 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?growth
  • 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

Flow cytometry can be used to measure the mass of DNA in a sample of cells throughout a cell cycle.

• It is essential that the DNA within new cells is accurate in order for them to carry out their function
• When the DNA is replicated (during the S phase) errors can occur
• There are several?checkpoints?throughout the cell cycle where the genetic information contained within the replicated?DNA is checked for any possible errors
• Specific?proof-reading enzymes?and?repair enzymes?are involved in this checking process
• When possible enzymes will repair the error but in some cases the?cell may destroy itself?to prevent passing on harmful mutations
• There are four checkpoints in the cell cycle:

1. During G1?phase?- chromosomes are checked for damage. If damage is detected then the cell does not advance into the S phase until repairs have been made
2. During S phase?- chromosomes are checked to ensure they have been replicated. If all the chromosomes haven't been successfully replicated then the cell cycle stops
3. During G2?phase?- an additional check for DNA damage occurs after the DNA has been replicated. The cell cycle will be delayed until any necessary repairs are made
4. During metaphase?- the final check determines whether the chromosomes are correctly attached to the spindle fibres prior to anaphase