In the resting (G0/G1) state of the cell cycle, the retinoblastoma protein prevents entry into S phase by which mechanism?
- A Ubiquitinating cyclin E for proteasomal degradation
- B Phosphorylating CDK4 at an inhibitory site
- C Binding and sequestering the transcription factor E2F ✓
- D Directly blocking the origin recognition complex at replication origins
Correct answer: C. Binding and sequestering the transcription factor E2F
Explanation
Hypophosphorylated Rb binds E2F transcription factors and keeps them inactive, preventing transcription of genes needed for S phase such as cyclin E and DNA polymerase. When cyclin D-CDK4/6 and cyclin E-CDK2 phosphorylate Rb, it releases E2F and the cell enters S phase. Rb does not degrade cyclins, phosphorylate CDKs, or act directly at replication origins.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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