Which of the following statements is true of DNA damage?

Question

BlackTom AI · Accepted Answer

Most DNA damage is repaired by the cell because cells possess multiple, highly conserved DNA repair systems that correct the bulk of lesions arising daily.

1) Relevant knowledge points and concepts
- DNA damage sources: endogenous (replication errors, spontaneous base hydrolysis, oxidative damage by reactive oxygen species) and exogenous (UV light, ionizing radiation, chemicals, biological agents). Many endogenous events are spontaneous chemical reactions.
- Major repair pathways: base excision repair (BER) for small base lesions, nucleotide excision repair (NER) for bulky adducts/UV photoproducts, mismatch repair (MMR) for replication errors, and double-strand break repair by homologous recombination (HR) or non-homologous end joining (NHEJ).
- Cellular responses: cell-cycle checkpoints, apoptosis, and senescence remove heavily damaged cells.

2) Step-by-step reasoning
- A typical human cell sustains on the order of 10^3–10^5 DNA lesions per day (commonly cited ≈10,000). 
- Step 1: Lesions are detected by sensor proteins (e.g., MutS/MutL for mismatches, XPC for bulky lesions).
- Step 2: Appropriate pathway recruited (BER, NER, MMR, HR/NHEJ) and lesion removed and gap filled by polymerases; ligases seal backbone.
- Step 3: If damage is excessive or irreparable, checkpoints trigger apoptosis or senescence to prevent propagation.

3) Example calculation (illustrative)
- If a cell acquires ~10,000 lesions/day and repair pathways successfully correct >99% of routine single-base lesions, then repaired lesions ≈ 9,900+, leaving only a small fraction unrepaired or misrepaired that can become mutations.

4) Why the correct answer is correct
- Because of the robust, efficient repair machinery, most lesions are fixed before they cause lasting mutations or disease. This is the basis for genomic stability in healthy cells.

5) Why other options are incorrect
- "Most DNA damage is advantageous": false — damage is usually neutral or harmful; beneficial changes are rare and subject to selection.
- "All DNA damage is caused by physical, chemical or biological agents": misleading—many lesions are spontaneous endogenous chemical events (e.g., depurination, deamination, replication errors), not always external agents.
- "All DNA damage results in diseases such as cancer": false — most damage is repaired; only a small subset leads to mutations that contribute to disease.

6) Conclusion
These points confirm the supplied correct choice: "Most DNA damage is repaired by the cell."

Which of the following statements is true of DNA damage?Single choice

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