Cell Cycle Checkpoints and a Chemotherapy Drug
A chemotherapy drug, compound M, is tested on cultured human cells. Flow cytometry shows that after 24 hours of treatment, the proportion of cells in S phase falls from 35% to 8%, while the proportion in G2/M rises from 15% to 52%. A marker of DNA double-strand breaks (γ-H2AX foci) increases fivefold within 6 hours of treatment. Cells with a defective G2/M checkpoint (mutant line) continue through mitosis despite the damage and show a much higher rate of chromosome fragmentation and death. Normal cells arrest and either repair or undergo apoptosis. The drug is also tested on a rapidly dividing non-cancerous tissue, which shows similar arrest and a measurable rise in apoptosis.
1. Identify which cell cycle phase compound M most likely targets and explain how the flow cytometry and DNA damage data together support your answer.
Read the worked answer
Target: S phase (DNA replication).
Reasoning:
- S-phase proportion collapses (35% → 8%): cells are not progressing through or completing replication.
- G2/M proportion rises (15% → 52%): cells that were already past S phase accumulate at the G2/M boundary, and cells attempting to enter S are blocked, so the population piles up before mitosis.
- γ-H2AX foci rise fivefold within 6 hours: this is a marker of DNA double-strand breaks, indicating the drug damages DNA. Damage arising during replication, with subsequent arrest before mitosis, points to an S-phase acting agent (for example, one that interferes with replication fork progression or damages DNA during synthesis).
The judgement: the phase distribution alone could be explained by a G2/M block, but the early DNA damage marker and the S-phase depletion together indicate the primary lesion is in replication, with G2/M accumulation as a downstream consequence of checkpoint arrest.
2. Explain the role of the G2/M checkpoint using the mutant cell line, and discuss one implication of the drug's effect on non-cancerous dividing tissue.
Read the worked answer
G2/M checkpoint role: it monitors DNA for damage and incomplete replication before mitosis. If damage is detected, the cell cycle is halted so repair can occur; if repair fails, apoptosis is triggered.
Mutant line evidence: cells lacking a functional checkpoint pass through mitosis with damaged DNA. The result is chromosome fragmentation and death - demonstrating that the checkpoint's function is protective. Without the arrest, damaged chromosomes are segregated into daughter cells, producing genetic instability. This is exactly the logic behind therapeutic strategies that combine DNA-damaging drugs with checkpoint inhibitors to push cancer cells into mitotic catastrophe.
Implication for non-cancerous dividing tissue: the drug does not distinguish cancer cells from normal rapidly dividing cells. The non-cancerous tissue shows arrest and increased apoptosis, which predicts side effects in tissues with high turnover - bone marrow, gut lining, hair follicles. This is the basis of the therapeutic window: dose must be tuned so tumour kill exceeds normal-tissue damage, and it explains why side effects are predictable from cell biology rather than random. A strong answer links the mechanism to the clinical trade-off rather than simply naming side effects.