10.3 - Control of the Cell Cycle

• Cyclins (proteins)

  • there is a mechanism for cell-cycle control that coordinates the timing of events in the cycle
  • similar across all eukaryotic cells
  • prevents segregation of cells that are still replicating
  • prevents cells from making too many copies of their DNA
  • cyclins are proteins that regulate progression through the cell cycle
• There are two types of regulatory cyclins:

  1. internal regulators: proteins that respond to control the cell cycle based on events in the cell - like DNA replication or spindle formation
  2. external regulators: proteins that respond to events outside the cell: stimulate growth and development - ex. during embryonic development & wound healing
• Checkpoint Controls

  • If something goes wrong in the cell cycle, what stops the cell cycle?
  • Examples of problems:
  • UV radiation may damage DNA
  • chromatids may fail to attach to the spindle
  • DNA base pairs may be mismatched
• Checkpoints ensure that replication errors and division errors are corrected before the cell cycle progresses.

  • holds the cell in cell cycle arrest
  • when checkpoint controls are damaged (these proteins are mutated), the cell cycle will not be regulated
  • ex: p53 - protein that detects mismatched base pairs; G1 cyclins are held off so the cell does not enter S until the repair is made.
  • Many cancer cells have a defective p53 gene - does not halt cell cycle until DNA is all replicated.
• Cancer: the body's cells lose the ability to control their cell cycle - the result is a tumor that damages surrounding tissue

  • happens when checkpoint controls are damaged
  • cancer cells divide uncontrollably and form a tumor
  • there are two types of tumors:
  • benign - tumor stays in one place
  • malignant - tumor spreads and metastasizes