Which of the following is not true about DNA double strand breaks?

Question

BlackTom AI · Accepted Answer

DNA double-strand breaks (DSBs) can be both damaging and physiologically useful depending on context. Key concepts: sources of DSBs (exogenous—ionizing radiation, chemicals; endogenous—replication stress, reactive oxygen species), cellular repair pathways (non-homologous end joining, NHEJ; homologous recombination, HR), and physiological roles of programmed DSBs (meiosis, V(D)J recombination, class-switch recombination).

Step-by-step reasoning:
1. DSBs and cancer (option a): Unrepaired or misrepaired DSBs can produce deletions, translocations, or oncogene activation (e.g., BCR-ABL fusion from translocation), driving tumorigenesis. Thus statement a is true.
2. DSBs and evolution (option b): During meiosis, the enzyme Spo11 creates programmed DSBs that are repaired by HR to produce crossovers and genetic recombination; this generates genetic diversity on which natural selection acts. Therefore b is true.
3. DSBs in immune response (option c): Developing B and T lymphocytes intentionally generate DSBs via RAG1/2 (V(D)J recombination) and AID-mediated lesions follow-up for class-switch recombination. These programmed breaks are essential for antibody and T-cell receptor diversity, so c is true.
4. Are DSBs always harmful (option d)? Not correct: many DSBs are deliberately induced and are essential for normal biology (meiosis, adaptive immunity). Moreover, cells possess dedicated repair mechanisms (HR in S/G2, NHEJ in G1) to fix DSBs. While accidental DSBs often harm cells, the blanket statement that DSBs are always harmful is false.

Why d is correct as the “not true” choice: options a–c describe well-established harmful and beneficial roles of DSBs; only d overgeneralizes by asserting all DSBs are harmful, which ignores physiological programmed DSBs and efficient repair systems. Conclusion: the correct answer is d.

Which of the following is not true about DNA double strand breaks?单项选择题

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