Cell Cycle

Cell cycle phases

  • Checkpoints control transitions between phases of cell cycle. This process is regulated by cyclins, cyclin-dependent kinases (CDKs), and tumor suppressors. M phase (shortest phase of cell cycle) includes mitosis (prophase, prometaphase, metaphase, anaphase, telophase) and cytokinesis (cytoplasm splits in two). G1 and G0 are of variable duration.

Regulation of cell cycle

  • Cyclin-dependent kinases
    • Constitutive and inactive.
  • Cyclins
    • Regulatory proteins that control cell cycle events; phase specific; activate CDKs.
  • Cyclin-CDK complexes
    • Phosphorylate other proteins to coordinate cell cycle progression; must be activated and inactivated at appropriate times for cell cycle to progress.
  • Tumor suppressors
    • p53 induces p21, which inhibits CDKs → hypophosphorylation (activation) of Rb →  inhibition of G1-S progression. Mutations in tumor suppressor genes can result in unrestrained cell division (eg, Li-Fraumeni syndrome).
    • Growth factors (eg, insulin, PDGF, EPO, EGF) bind tyrosine kinase receptors to transition the cell from G1 to S phase.

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