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Cell Cycle Clock (Cell Cycle Control)

Definition

The cell cycle clock is a regulatory system that controls the timing and progression of the cell cycle. It ensures cells divide accurately, efficiently, and only when appropriate.

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Purpose
• Ensures DNA is fully replicated before division
• Prevents damaged or abnormal cells from dividing
• Coordinates cell growth, DNA replication, and mitosis

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Main Components

1. Cyclins
• Regulatory proteins whose concentration rises and falls during the cell cycle
• Activate cyclin-dependent kinases (CDKs)
• Different cyclins work at different stages (G1, S, G2, M)

2. Cyclin-Dependent Kinases (CDKs)
• Enzymes that trigger cell cycle events when bound to cyclins
• Present constantly in the cell but only active when cyclin is attached

3. Cyclin-CDK Complex
• When cyclin binds to CDK, the complex:
• Phosphorylates target proteins
• Pushes the cell to the next phase of the cell cycle

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Cell Cycle Checkpoints

1. G1 Checkpoint (Restriction Point)
• Checks:
• Cell size
• Nutrients
• DNA damage
• If conditions are unfavorable → cell enters G0 (resting phase)

2. G2 Checkpoint
• Checks:
• DNA replication is complete
• DNA damage repaired
• Prevents entry into mitosis if problems exist

3. M Checkpoint (Spindle Checkpoint)
• Ensures chromosomes are properly attached to spindle fibers
• Prevents unequal chromosome separation

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Role of p53 (Tumor Suppressor Protein)
• Detects DNA damage
• Can:
• Pause the cell cycle for repair
• Trigger apoptosis (programmed cell death) if damage is severe
• Often mutated in cancer cells

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Importance
• Maintains genetic stability
• Prevents uncontrolled cell division
• Malfunction can lead to cancer

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Summary
• Cell cycle clock = cyclins + CDKs
• Operates through checkpoints
• Ensures safe and controlled cell division