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.
- 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.