Learning Objectives

• Describe the phases of the cell cycle and their relative durations.
• Explain the roles of Cyclins, CDKs, and Tumor Suppressors in cell cycle control.
• Master the p53/p21/Rb pathway and its clinical significance.
• Differentiate between Permanent, Stable, and Labile cell types.

1. Phases of the 

The cell cycle consists of Interphase (G1, S, G2) and Mitosis (M).

• G1 (Gap 1): Variable duration; cell grows and prepares for DNA synthesis.
• S (Synthesis): DNA replication occurs.
• G2 (Gap 2): Preparation for mitosis.
• M (Mitosis): The shortest phase. Includes.
• G0: A quiescent state where the cell is not actively dividing.

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2. Regulation & Checkpoints

Transitions are strictly controlled to prevent errors in DNA replication or division. These processes are regulated by Cyclins and Cyclin-Dependent Kinases (CDKs).

• CDKs: Constitutively expressed but inactive when not bound to cyclin.
• Cyclins: Phase-specific regulatory proteins that activate CDKs when stimulated by growth factors.
• Cyclin-CDK Complex: Phosphorylates target proteins (like Rb) to coordinate cell cycle progression.

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3. Tumor Suppressors & The G1-S Brake

The transition from G1 to S phase is a major regulatory hurdle often mediated by growth factors (e.g., insulin, PDGF, EPO, EGF) binding tyrosine kinase receptors.

Clinical High-Yield: Mutations in tumor suppressor genes can result in unrestrained cell division, as seen in Li-Fraumeni syndrome.

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4. Cell Types and Proliferative Capacity

Cells vary in their ability to regenerate and their position within the cell cycle.

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