The Cell Cycle

• doubling of its genome (DNA) in S phase (synthesis phase) of the cell cycle
• halving of that genome during mitosis (M phase)

The period between M and S is called G₁; that between S and M is G₂.

• G₁ = growth and preparation of the chromosomes for replication
• S = synthesis of DNA (and centrioles) [see DNA Replication]
• G₂ = preparation for
• M = mitosis

Control of the Cell Cycle

• Cyclins There are 3 groups:
  • G₁ cyclins
  • S-phase cyclins
  • M-phase cyclins
  Their levels in the cell rise and fall with the stages of the cell cycle.
• Cyclin-dependent kinases (CDKs) Again, there are 3 groups:
  • G₁ CDKs
  • S-phase CDKs
  • M-phase CDKs
  Their levels in the cell remain fairly stable, but each must bind the appropriate cyclin (whose levels fluctuate) in order to be activated. They add phosphate groups to a variety of protein substrates that control processes in the cell cycle.
• The anaphase-promoting complex (APC) and other proteolytic enzymes. APC degrades the mitotic cyclins.

Steps in the cycle

• a rising level of G₁ cyclins signals the cell to prepare the chromosomes for replication
• a rising level of S-phase promoting factor prepares the cell to enter S phase and duplicate its DNA (and its centrioles)
• as DNA replication continues, one of the cyclins shared by G₁ and S-phase CDKs (cyclin E) is destroyed and the level of mitotic cyclins begins to rise (in G₂)
• M-phase promoting factor (the complex of mitotic cyclins with M-phase CDK) initiates

  assembly of the mitotic spindle

  breakdown of the nuclear envelope

  condensation of the chromosomes

• these events take the cell to metaphase of mitosis
• at this point, the M-phase promoting factor activates the anaphase promoting complex (APC) which
  • separates the sister chromatids at the metaphase plate (= anaphase), completing mitosis
  • destroys the M-phase cyclins
  • turns on synthesis of G₁ cyclins for the next turn of the cycle.

G₀

Many times a cell will leave the cell cycle, temporarily or permanently. It exits the cycle at G₁ and enters a stage designated G₀ (G zero). A G₀ cell is often called "quiescent", but that is probably more a reflection of the interests of the scientists studying the cell cycle than the cell itself. Many G₀ cells are anything but quiescent. They are busy carrying out their functions in the organism. e.g., secretion, conducting nerve impulses, attacking pathogens.

Often G₀ cells are terminally differentiated: they will never reenter the cell cycle but instead will carry out their function in the organism until they die.

For other cells, G₀ can be followed by reentry into the cell cycle. Most of the lymphocytes in human blood are in G₀. However, with proper stimulation, such as encountering the appropriate antigen, they can be stimulated to reenter the cell cycle (at G₁) and proceed on to new rounds of alternating S phases and mitosis.