## Why option 2 is correct Bleomycin's cytotoxic mechanism relies on generating DNA strand breaks via Fe²⁺-dependent oxidative cleavage. These breaks accumulate on both strands (including the nicks between Okazaki fragments on the lagging strand) faster than DNA ligase (marked **D**) can seal them by forming phosphodiester bonds. The overwhelming burden of damage exceeds the cell's repair capacity, triggering apoptosis. This is the basis for bleomycin's use in Hodgkin lymphoma and testicular cancer. The drug is cell-cycle-specific (G2 phase), when replication fork activity is high and ligase demand is greatest (Harper 32e Ch 35; KD Tripathi 9e Ch 62). ## Why each distractor is wrong - **Option 1**: Bleomycin does not directly inhibit DNA ligase. It acts upstream by creating the DNA damage (strand breaks) that ligase must repair. Ligase itself remains functional but becomes rate-limiting. - **Option 3**: Bleomycin does not inhibit primase. Its mechanism is oxidative strand cleavage, not inhibition of primer synthesis. Even if primers were absent, this would not explain bleomycin's cytotoxicity. - **Option 4**: Bleomycin does not activate helicase. Helicase (marked **A**) unwinds DNA normally; bleomycin's damage occurs after unwinding, as oxidative cleavage of the exposed strands. **High-Yield:** Bleomycin toxicity = DNA damage rate >> DNA ligase repair rate → cell death. Major clinical toxicity: pulmonary fibrosis (lungs lack bleomycin hydrolase, allowing drug accumulation). [cite: Harper 32e Ch 35; KD Tripathi 9e Ch 62]
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