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Cell cycle and apoptosis · USMLE Step 1

What is the primary role of cyclin-dependent kinases (CDKs) in the eukaryotic cell cycle?

  1. They directly replicate DNA molecules during S phase
  2. They degrade cyclins in order to allow cell cycle progression
  3. They phosphorylate target proteins to trigger phase transitions
  4. They act as transcription factors for cell-cycle genes
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Correct answer: They phosphorylate target proteins to trigger phase transitions

CDKs, when bound to their cyclin partners, phosphorylate target proteins that promote progression from one phase to the next (e.g., G1/S or G2/M transitions), thereby regulating the orderly cell cycle.

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