2.6.1 The 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 = 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