Which statement about p53 is true?

Which statement about p53 is true?

• A. It is a transcription factor for p21, blocking G1-S progression
• B. It activates cyclin D to promote cell cycle
• C. It is a growth factor receptor
• D. It degrades PTEN

Answer: (A)

The key idea is that p53 acts as a tumor suppressor transcription factor that halts the cell cycle in response to stress. One of its main targets is p21 (a CDK inhibitor). When p53 increases p21 expression, p21 binds to and inhibits cyclin-CDK complexes, especially those needed for the G1 to S transition. This prevents phosphorylation of the retinoblastoma (Rb) protein, so E2F remains repressed and cells cannot enter S phase. In short, p53-mediated activation of p21 stops the cell cycle to allow DNA repair; if damage is too severe, p53 can also trigger apoptosis. The other statements don’t fit with p53’s role: it does not promote cyclin D or drive cell-cycle progression, and it is not a growth factor receptor. It also does not degrade PTEN.

The key idea is that p53 acts as a tumor suppressor transcription factor that halts the cell cycle in response to stress. One of its main targets is p21 (a CDK inhibitor). When p53 increases p21 expression, p21 binds to and inhibits cyclin-CDK complexes, especially those needed for the G1 to S transition. This prevents phosphorylation of the retinoblastoma (Rb) protein, so E2F remains repressed and cells cannot enter S phase. In short, p53-mediated activation of p21 stops the cell cycle to allow DNA repair; if damage is too severe, p53 can also trigger apoptosis.

The other statements don’t fit with p53’s role: it does not promote cyclin D or drive cell-cycle progression, and it is not a growth factor receptor. It also does not degrade PTEN.