Regulation of the Cell Cycle (College Board AP Biology)

• The cell cycle is a sequence of stages including interphase (G₁, S & G), mitosis (M) and cytokinesis (C)
• There are three checkpoints in the cell cycle which must be overridden before the next stage can begin
  • These checkpoints are located at G₁, G₂ and M 
• The cycle is controlled by cyclins (a group of proteins) and kinases (enzymes)
• There are four different cyclins (D, E, A & B) whose concentrations rise and fall over the cycle:
  • Each of these will trigger specific events in the cell cycle to occur

Cyclin control of the cell cycle diagram

Cyclins control the cell cycle. The presence of certain cyclins triggers a specific stage of the cell cycle.

• When each of the different cyclins reach a certain concentration (or threshold level) they trigger the next stage of the cell cycle
• This ensures key processes (e.g. DNA replication, organelle multiplication and protein synthesis) occur at the correct time
• When a specific cyclin has reached a certain concentration it will bind with another group of proteins (cyclin-dependent kinases) forming a complex which is activated
• This complex phosphorylates (attaches a phosphate) a target protein which activates it, causing it to trigger specific functions (e.g. DNA replication)
• Once the specific function is complete the phosphate is released, the cyclin breaks down and the cyclin-dependent kinases become inactive

Mechanism of cyclin action diagram

The mechanism for the cell cycle control by cyclins

• Mutations of the genes that control the cell cycle may lead to cancer or apoptosis (programmed cell death)

Apoptosis

• Apoptosis is a mechanism used to protect the body from mutated cells which are damaged beyond repair
• It plays a role in preventing cancer
• Apoptosis is triggered by a series of controlled biochemical events in which enzymes catalyse the breakdown of cellular components and eventually the entire cell

Cancer

• Two types of genes are typically involved with the development of cancer:
  • Proto-oncogenes code for proteins which stimulate normal cell division
    • These genes are converted to oncogenes once they mutate
    • This results in an increase in the protein product produced or proteins which are permanently activated
    • Leading to uncontrolled cell division which may result in cancer
  • Tumor suppressor genes code for proteins which inhibit cell division or promote controlled cell death (known as apoptosis) should the nucleus contain damaged DNA
    • Mutations in these genes may result in no or reduced protein product or proteins which are permanently deactivated
    • Leading to uncontrolled cell division and possibly cancer