Regulation of the cell cycle: checkpoints, cyclins, CDKs, cancer and apoptosis

• Section I (Multiple Choice): Expect stimulus sets featuring concentration graphs of cyclins and cyclin-dependent kinases (CDKs) over time, or experimental data measuring the percentage of cells in each phase after treatment with a putative anti-cancer drug.
• Section II (Free Response): Frequently appears in Q4 (Conceptual Analysis) or Q5 (Analyze Model/Visual Representation), where you must describe how a checkpoint works, explain how cyclin–CDK interactions drive phase transitions, and predict or justify the outcome of a disrupted signaling pathway.

Core ideas

Internal checkpoints

Checkpoints are internal control mechanisms at critical stages of the cell cycle that verify whether the cell is ready to progress to the next phase.

Cyclins and cyclin-dependent kinases (CDKs)

Progression past checkpoints is driven by molecular interactions between two families of regulatory proteins:

1. Cyclin-dependent kinases (CDKs): Enzymes that phosphorylate target proteins to trigger downstream cell-cycle events (e.g., chromosome condensation or nuclear envelope breakdown). CDKs are present at a constant concentration throughout the cell cycle, but they are inactive on their own.
2. Cyclins: Regulatory proteins whose concentrations fluctuate cyclically throughout the cell cycle. Cyclins are synthesized in response to internal or external signals and are rapidly degraded by cytoplasmic proteases once their phase transition is complete.

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