Which statement best describes the function of the RB1 (retinoblastoma) protein in normal cell cycle regulation?

• A It phosphorylates cyclin-dependent kinases to promote S-phase entry
• B It sequesters E2F transcription factors, preventing transcription of S-phase genes ✓
• C It degrades p21 to allow cyclin D-CDK4 complex formation
• D It activates the proteasomal degradation of p53

Explanation

Unphosphorylated (active) RB1 binds and sequesters E2F transcription factors, preventing transcription of genes needed for S-phase entry. Growth factor signaling activates cyclin D-CDK4/6, which phosphorylates RB, releasing E2F and allowing cell cycle progression past the G1/S checkpoint. Loss of RB function (mutation or viral oncoprotein binding) removes this brake on proliferation.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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