Questions: DNA Repair Mechanisms

Question 1

Which DNA repair pathway specifically recognizes and removes UV-induced thymine dimers?

• A. Mismatch repair (MMR)
• B. Base excision repair (BER)
• C. Nucleotide excision repair (NER)
• D. Non-homologous end joining (NHEJ)

Answer: (C)

Thymine dimers are bulky helix-distorting lesions caused by UV cross-linking of adjacent thymines. NER recognizes helix distortion rather than specific chemical damage, cuts out a ~25-30 nucleotide patch surrounding the lesion, and resynthesizes using the complementary strand. Xeroderma pigmentosum results from NER defects.

Question 2

Non-homologous end joining (NHEJ) is a high-fidelity repair mechanism that perfectly restores the original DNA sequence at a double-strand break.

• T. True
• F. False

Answer: False

NHEJ is error-prone: it ligates broken ends directly without a homologous template, often introducing small insertions or deletions at the junction. Unlike homologous recombination, which uses the sister chromatid as a high-fidelity guide, NHEJ trades accuracy for availability — it operates throughout the cell cycle but at the cost of sequence integrity.

Question 3

Why is homologous recombination considered higher-fidelity repair than non-homologous end joining for double-strand breaks?

Model answer: Homologous recombination uses the intact sister chromatid as a template, allowing the broken sequence to be copied back accurately. NHEJ ligates broken ends without a template, so small insertions or deletions at the join are common.

The fidelity of HR comes from copying information from an identical intact molecule. NHEJ has no such reference, so it re-joins whatever ends are available — sometimes imprecisely. This is why HR is preferred in S/G2 phase when sister chromatids are available, while NHEJ is the fallback in G1.