Questions: Cell Cycle Checkpoints: Ensuring Genome Integrity

Question 1

A researcher treats dividing cells with a drug that permanently activates p21 in all cells. What would be the expected outcome?

• A. Cells would divide more rapidly because p21 enhances cyclin-Cdk signaling
• B. Cell cycle progression would permanently halt because active p21 inhibits Cdk activity, blocking the G1/S transition
• C. Cells would immediately undergo apoptosis because p21 directly activates the apoptosis program
• D. The drug would have no effect because p21 only functions transiently during DNA damage responses

Answer: (B)

p21 is a Cdk inhibitor — it binds and inactivates cyclin-Cdk complexes, preventing them from phosphorylating their targets. At the G1/S checkpoint, Rb must be phosphorylated by cyclin D-Cdk4/6 and cyclin E-Cdk2 to release E2F and allow S-phase entry. Permanent p21 activation locks Rb in its unphosphorylated, repressive state and blocks the transition. p21 does not directly trigger apoptosis — that requires p53 activating separate pro-apoptotic targets when damage is irreparable, and only after arrest and repair have been attempted first.

Question 2

Why does cancer typically require multiple mutations to develop rather than arising from a single mutation?

• A. Single mutations are too small to alter protein function significantly
• B. Cancer cells evolve immune evasion before a second mutation can take hold
• C. Cells have redundant checkpoint mechanisms, so multiple independent layers must be disabled before cells can divide without restraint
• D. The immune system reliably destroys cells with single checkpoint mutations before they can proliferate

Answer: (C)

The cell cycle has multiple independent checkpoints — G1/S, G2/M, and the spindle assembly checkpoint — plus additional redundancy within each. Loss of p53 disables the DNA damage response but does not eliminate Rb-mediated G1 control or spindle checkpoint surveillance. Each additional mutation that disables another layer removes a further barrier. This is why carcinogenesis is a multi-step process requiring mutation accumulation over years. Conversely, this redundancy means most cells with isolated checkpoint failures do not become cancerous.

Question 3

The spindle assembly checkpoint prevents chromosome separation as long as even one kinetochore remains unattached to spindle fibers from both poles.

• T. True
• F. False

Answer: True

This is one of the most stringent checkpoint mechanisms in cell biology. Even a single unattached kinetochore generates a potent wait signal — the mitotic checkpoint complex inhibits the anaphase-promoting complex, preventing securin degradation and sister chromatid separation. This ensures all chromosomes are correctly attached before the irreversible division step proceeds. Bypass of this checkpoint leads to aneuploidy — the wrong chromosome number in daughter cells — which is a hallmark of cancer cells.

Question 4

When p53 detects DNA damage, its primary response is to trigger immediate apoptosis in order to eliminate the dangerous cell as quickly as possible.

• T. True
• F. False

Answer: False

p53 primarily activates cell cycle arrest first, via transcription of p21 (a Cdk inhibitor), giving DNA repair enzymes time to fix the damage. Apoptosis is p53's response when damage is too extensive to repair — it is a last resort, not the first action. This hierarchy (arrest, attempt repair, then apoptosis if irreparable) reflects the cell's priority: saving a repairable cell is better than eliminating it. The common misconception that damaged DNA always triggers death ignores the normal repair-first strategy.

Question 5

Why is p53 called the guardian of the genome, and what happens to this guardianship when p53 is mutated in cancer?

Model answer: p53 monitors for DNA damage: when sensor kinases detect damage, p53 is stabilized and activates genes for cell cycle arrest (via p21) and DNA repair, or apoptosis if damage is irreparable. This prevents damaged cells from proliferating and transmitting mutations to daughter cells. When p53 is mutated, this surveillance is lost: damaged cells pass through checkpoints without repair, accumulate additional mutations, and can eventually acquire all the hallmarks of malignancy. This is why p53 is the most commonly mutated gene in human cancers.

p53 is described as a guardian rather than just a tumor suppressor because its function is proactive surveillance rather than passive blocking. It continuously monitors cellular integrity and responds proportionately — arrest for repairable damage, apoptosis for irreparable damage. Its mutation in more than half of all human cancers illustrates how central this guardianship is: without it, the other redundant checkpoint mechanisms are progressively overwhelmed as additional mutations accumulate over time.