p53 in cancer biology: Over 50% of common human cancers harbor defects in p53. Which classification best describes the normal p53 protein?

Introduction / Context:
p53 is often called the “guardian of the genome.” Understanding its role clarifies why mutations in TP53 are so prevalent across cancers and how cells balance repair with apoptosis when DNA damage occurs.

Given Data / Assumptions:

• p53 monitors DNA damage and cellular stress.
• Activation can cause cell cycle arrest, senescence, or apoptosis.
• We must select the correct functional class.

Concept / Approach:
A tumor suppressor prevents uncontrolled proliferation by enforcing checkpoints and enabling repair. p53 acts mainly as a transcription factor inducing genes such as p21 (CDK inhibitor) to arrest the cycle and others that promote DNA repair or apoptosis. Loss-of-function mutations disable these safeguards, facilitating oncogenesis.

Step-by-Step Solution:
Identify p53's overarching role: restrain division under stress.Match to category: tumor suppressor (not growth promoter).Therefore, select “a tumor suppressor.”

Verification / Alternative check:
Knockout models and human tumors with TP53 mutations show genomic instability and resistance to apoptosis, hallmarks of tumor suppressor loss.

Why Other Options Are Wrong:

• Cyclin: p53 is not a cyclin and does not activate CDKs.
• Oncogene: oncogenes promote growth when activated; p53 restrains growth when functional.
• Receptor tyrosine kinase: p53 is a nuclear transcription factor, not a membrane receptor.

Common Pitfalls:
Confusing “tumor suppressor gene” with “oncogene.” They have opposite effects and mutation patterns (loss-of-function vs gain-of-function).

Final Answer:
a tumor suppressor