Questions: DNA Damage Detection and Checkpoint Responses

Question 1

In a healthy, undamaged cell, p53 protein levels are kept very low. Which mechanism is responsible for this, and why does it matter for understanding how checkpoint signaling works?

• A. The p53 gene is only transcribed in response to damage signals; in undamaged cells, no p53 mRNA is produced
• B. MDM2 continuously ubiquitinates p53, targeting it for proteasomal degradation; DNA damage disrupts this interaction, allowing p53 to accumulate and function
• C. p53 is sequestered in the cytoplasm by chaperone proteins and only enters the nucleus after ATM phosphorylation
• D. MicroRNAs degrade p53 mRNA in undamaged cells; ATR activation inhibits these microRNAs

Answer: (B)

The MDM2-p53 feedback loop is the key mechanism. In normal cells, p53 is continuously produced but also continuously destroyed by MDM2, keeping its levels low. When ATM or ATR phosphorylate p53 (and also MDM2), this disrupts the p53-MDM2 interaction — MDM2 can no longer efficiently tag p53 for degradation. P53 rapidly accumulates and functions as a transcription factor. This design means the cell can respond very quickly to damage without waiting for new gene transcription, since the system works by stabilizing a protein that is always being made rather than by inducing a new gene.

Question 2

A rapidly dividing cell encounters a chemical mutagen that stalls multiple replication forks by creating bulky DNA adducts. Which sensor kinase is primarily activated, and what distinguishes its activation signal from that of the other major checkpoint kinase?

• A. ATM — it recognizes double-strand breaks through the MRN complex binding to broken chromosome ends
• B. ATR — it recognizes single-stranded DNA coated with RPA, which accumulates at stalled replication forks, distinct from ATM which responds to double-strand breaks
• C. Chk1 — it directly senses replication fork stalling through interaction with PCNA
• D. p53 — it acts as both the sensor and the effector for replication stress

Answer: (B)

ATM and ATR respond to fundamentally different damage signals. ATM is activated by double-strand breaks (DSBs) — the MRN complex binds the broken ends and recruits ATM. ATR is activated by single-stranded DNA coated with the replication protein RPA, which accumulates when replication forks stall (due to bulky adducts, crosslinks, or other replication-blocking lesions). Stalled replication forks produce RPA-coated ssDNA as the helicase continues to unwind DNA while the polymerase is blocked. ATR is recruited to this structure via ATRIP. Think of ATM as the broken-chromosome alarm and ATR as the stalled-replication alarm.

Question 3

When p53 is activated by DNA damage, it usually triggers apoptosis immediately to prevent the damaged cell from dividing and passing mutations to daughter cells.

• T. True
• F. False

Answer: False

P53's response is graded by the severity and repairability of the damage, not a simple on/off apoptosis switch. For repairable damage, p53's primary initial response is to induce p21, a CDK inhibitor that halts cell cycle progression — buying time for DNA repair enzymes to work. P53 also upregulates DNA repair genes. Apoptosis (via Bax, PUMA, Noxa) is reserved for cases where damage is assessed as irreparable, because killing a cell is irreversible. Triggering immediate apoptosis for all damage would destroy cells that could be repaired, which is itself harmful. The threshold between cell cycle arrest and apoptosis is set by the persistence and magnitude of checkpoint signaling.

Question 4

In a healthy cell, p53 protein is continuously produced but maintained at low levels because it is continuously degraded by MDM2.

• T. True
• F. False

Answer: True

This is an important mechanistic point often missed by students who think of p53 as a 'damage-inducible' protein. P53 is constitutively expressed — it is always being made — but MDM2 ubiquitinates it for proteasomal degradation just as fast, keeping steady-state levels low. When DNA damage occurs and ATM/ATR phosphorylate p53, this disrupts the MDM2 interaction, and p53 levels rise rapidly through reduced degradation rather than increased synthesis. This post-translational regulation allows the cell to respond within minutes, not hours.

Question 5

Explain why p53 is often called the 'guardian of the genome,' and describe what happens to cells that sustain irreparable DNA damage when p53 is functional versus when p53 is mutated.

Model answer: P53 is called the guardian of the genome because it is the central effector of the DNA damage checkpoint — detecting damage, halting the cell cycle to allow repair, and eliminating cells with irreparable damage. When damage is irreparable, functional p53 activates pro-apoptotic genes (Bax, PUMA, Noxa) or induces permanent senescence, removing the damaged cell from the proliferating pool and preventing mutation transmission. When p53 is mutated or lost, damaged cells continue cycling, accumulating additional mutations with each division — fueling tumor evolution. This explains why p53 is the most commonly mutated gene in human cancers (roughly half of all tumors).

The guardian-of-the-genome framing highlights p53's dual role: it both gives time for repair (via p21-mediated arrest) and enforces a final solution when repair fails (via apoptosis or senescence). Loss of p53 removes both safeguards simultaneously, which is why it is such a powerful driver of genomic instability and cancer progression.