Learning objective:
By the end of this lesson, students should be able to: Explain the phases of the cell cycle, describe how checkpoints are regulated by cyclins, CDKs, and tumor suppressors, and apply this understanding to clinical scenarios involving cancer pathogenesis.
Overview of the Cell Cycle
The cell cycle consists of distinct phases that ensure accurate DNA replication and division. Control mechanisms, including checkpoints and regulatory proteins, maintain genomic integrity.
| Phase | Key Events | Clinical Notes |
|---|---|---|
| G1 (Gap 1) | Cell growth, organelle duplication | Variable duration; critical checkpoint before DNA replication |
| S (Synthesis) | DNA replication | Targeted by anti-metabolite chemotherapeutics |
| G2 (Gap 2) | Preparation for mitosis, DNA repair | Checkpoint for DNA damage before mitosis |
| M (Mitosis) | Cell division (prophase → telophase + cytokinesis) | Shortest phase |
| G0 (Quiescent) | Resting phase | Non-dividing cells like neurons remain here permanently |
Regulation of the Cell Cycle
- Cyclin-Dependent Kinases (CDKs)
- Constitutively expressed but inactive unless bound to cyclins.
- Phosphorylate target proteins to regulate phase transitions.
- Cyclins
- Phase-specific proteins that activate CDKs.
- Synthesis is stimulated by growth factors.
- Cyclin-CDK complexes coordinate transitions between phases (e.g., G1 → S).
- Tumor Suppressors
- p53
- Induces p21, inhibiting CDKs Mutation → unrestrained growth (e.g., Li-Fraumeni syndrome)
- Rb
- When hypophosphorylated, inhibits E2F, blocking G1→S Loss → uncontrolled cell cycle progression (e.g., retinoblastoma)
- p53
Growth Factors
- Growth factors like
- Insulin,
- PDGF,
- EPO, and
- EGF binds tyrosine kinase receptors, promoting progression from G1 → S.
Cell Types and Their Division Capacity
| Type | Cell Cycle Behavior | Examples | Clinical Relevance |
|---|---|---|---|
| Permanent | Remain in G0; do not divide | Neurons, skeletal & cardiac muscle, RBCs | Cannot regenerate after injury |
| Stable (Quiescent) | Enter G1 from G0 when stimulated | Hepatocytes, lymphocytes, PCT, periosteal cells | Regenerate when needed |
| Labile | Constantly dividing; no G0 | Bone marrow, gut epithelium, skin, hair follicles, germ cells | Most affected by chemotherapy |
Key Points for USMLE Step 1
- Cyclin-CDK complex drives phase transitions.
- p53 and Rb are major checkpoint guardians.
- Growth factors initiate the G1→S transition.
- Labile cells are highly mitotic and chemotherapy-sensitive.
- Mutations in tumor suppressors can cause cancers like retinoblastoma and Li-Fraumeni syndrome.
