AP Biology: Regulation by Cyclins and Cyclin?Dependent Kinases (CDKs)

Regulation by Cyclins and Cyclin?Dependent Kinases (CDKs)

Concept Summary

• Cyclins: Regulatory proteins that bind and activate CDKs, controlling progression through the cell cycle; their levels fluctuate cyclically.
• Cyclin-dependent kinases (CDKs): Enzymes that phosphorylate target proteins to drive cell cycle transitions; active only when bound to cyclins.
• Checkpoints: Surveillance mechanisms (e.g., G1, G2, M) where cyclin-CDK complexes enforce cell cycle arrest if conditions are unfavorable.
• Ubiquitin-mediated proteolysis: Process by which cyclins are degraded (e.g., via APC/C), ensuring irreversible cell cycle progression.
• Retinoblastoma protein (Rb): Key CDK target that inhibits E2F transcription factors until phosphorylated, releasing cells from G1 arrest.

Core Questions

WHAT (definitional)

Q: What is a cyclin? A: A regulatory protein whose concentration rises and falls to activate CDKs at specific cell cycle stages. Trap/Clarification: Cyclins do not have enzymatic activity—they activate CDKs, which phosphorylate targets.

Q: What is a CDK? A: A serine/threonine kinase that phosphorylates proteins to drive cell cycle events, active only when bound to a cyclin. Trap/Clarification: CDK levels remain constant; their activity depends on cyclin binding, not expression.

WHY (causal/explanatory)

Q: Why are cyclins degraded after their phase? A: To ensure unidirectional cell cycle progression—degradation inactivates CDKs, preventing reversion to earlier stages. Trap/Clarification: Cyclin degradation is irreversible; synthesis of new cyclins is required for the next cycle.

Q: Why is Rb phosphorylation by CDKs important? A: Phosphorylated Rb releases E2F, allowing transcription of S-phase genes (e.g., DNA polymerase) to initiate replication. Trap/Clarification: Rb is not a cyclin or CDK—it’s a CDK target that acts as a "brake" on the cell cycle.

HOW (process/application)

Q: How do cyclin-CDK complexes regulate the G1/S transition? A: G1 cyclins (e.g., cyclin D) bind CDK4/6-phosphorylate Rb-release E2F-activate S-phase gene transcription. Trap/Clarification: Cyclin D-CDK4/6 partially phosphorylates Rb; full phosphorylation requires cyclin E-CDK2.

Q: How is CDK activity inhibited? A: By CKIs (CDK inhibitors) like p21/p27, which bind cyclin-CDK complexes to block kinase activity, or by inhibitory phosphorylation (e.g., Wee1 kinase). Trap/Clarification: CKIs do not degrade cyclins—they inactivate the complex; cyclin degradation is separate (via APC/C).

CAN (conditions/possibilities)

Q: Can CDKs function without cyclins? A: No—CDKs are catalytically inactive without cyclin binding, which induces conformational changes to expose the active site. Trap/Clarification: Some CDKs (e.g., CDK1) can bind multiple cyclins (e.g., cyclin A/B), but all require a cyclin partner.

Q: Under what conditions does p53 activate p21? A: In response to DNA damage, p53 transcriptionally upregulates p21, which inhibits cyclin-CDK complexes to arrest the cell cycle. Trap/Clarification: p21 only inhibits CDKs—it does not repair DNA or degrade cyclins.

Quick Facts & Traps

• Fact: Cyclin-CDK specificity: Cyclin D-CDK4/6 (G1), cyclin E-CDK2 (G1/S), cyclin A-CDK2 (S), cyclin B-CDK1 (M).
• Trap: "CDK levels fluctuate"-Reality: Cyclin levels fluctuate; CDK levels are constant.
• Fact: APC/C (Anaphase-Promoting Complex/Cyclosome) tags cyclins (e.g., cyclin B) with ubiquitin for proteasomal degradation.
• Trap: "All cyclins are degraded by APC/C"-Reality: APC/C targets M-phase cyclins (e.g., cyclin B); G1/S cyclins (e.g., cyclin E) are degraded by SCF.
• Fact: Positive feedback loops: Active cyclin B-CDK1 phosphorylates Cdc25 (activating it) and inhibits Wee1 (reducing inhibitory phosphorylation).
• Trap: "CDKs only phosphorylate Rb"-Reality: CDKs phosphorylate many targets (e.g., lamins, condensins, histones).

Rapid-Fire True/False

• Statement: CDKs are active throughout the cell cycle. Answer: FALSE Why the common mistake happens: Students confuse constant CDK expression with fluctuating activity (which depends on cyclins).

• Statement: Cyclin degradation is reversible. Answer: FALSE Why the common mistake happens: Students assume cyclins can be "recycled" like enzymes, ignoring ubiquitin-mediated proteolysis.

• Statement: p21 inhibits CDKs by degrading cyclins. Answer: FALSE Why the common mistake happens: Students conflate CKIs (which bind/inactivate complexes) with ubiquitin ligases (which degrade cyclins).