4.7 Regulation of Cell Cycle

Checkpoints

The cell cycle is controlled at checkpoints (at G1, G2, and M) that verify the cell is ready to continue — DNA undamaged, fully replicated, chromosomes attached. If something is wrong, the cycle pauses. The G1 checkpoint is a major decision point about whether to divide.

Regulators and cancer

Checkpoints are run by regulatory proteins: cyclins and cyclin-dependent kinases (CDKs). External growth factors can signal a cell to divide. If these controls fail, cells divide uncontrollably — the basis of cancer. Damaged cells may instead undergo apoptosis (programmed cell death).

Key terms

Checkpoint

A control point (G1, G2, M) that verifies the cell is ready to continue the cycle.

Cyclin

A regulatory protein whose changing levels control cell cycle progression.

CDK

Cyclin-dependent kinase — activated by cyclins to drive the cycle forward.

Growth factor

An external signal protein that stimulates a cell to divide.

Apoptosis

Programmed cell death that removes damaged or unneeded cells.

Try an AP-style question

Why does Cdk activity rise and fall even though Cdk levels are constant?

1. Cyclin inhibits Cdk when cyclin levels are high
2. Cdk is active only when bound to cyclin
3. Cdk is destroyed and rebuilt each cycle
4. Cdk activity depends only on ATP levels