p53 tumour suppressor protein is called the 'guardian of the genome' because it:
- A Transcriptionally activates p21 (CIP1) to cause G1 arrest and promotes apoptosis via BAX upregulation when DNA damage is irreparable ✓
- B Directly repairs DNA double-strand breaks via homologous recombination
- C Phosphorylates RB protein to release E2F transcription factors
- D Inhibits telomerase to limit replicative lifespan
Correct answer: A. Transcriptionally activates p21 (CIP1) to cause G1 arrest and promotes apoptosis via BAX upregulation when DNA damage is irreparable
Explanation
After genotoxic stress, ATM/ATR kinases stabilise p53 by phosphorylating MDM2 (preventing p53 degradation); p53 then transcribes p21 (CDKN1A), a CDK inhibitor that arrests cells in G1 for repair, and if damage is irreparable, upregulates BAX and PUMA (pro-apoptotic BCL-2 family members). p53 does not directly repair DNA or phosphorylate Rb (CDK4/6 do that). Telomerase inhibition is mediated by other mechanisms.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.