Ultraviolet light can create thymine dimers in DNA. Among common repair routes, which mechanism is error prone and therefore most likely to introduce new mutations while allowing replication to proceed?

Introduction / Context:
Thymine dimers are a hallmark of UV damage that can block replication. Cells deploy several repair systems with different accuracies. Understanding which pathway is error prone helps explain mutagenesis after UV exposure and the balance between survival and genome integrity.

Given Data / Assumptions:

• DNA contains UV induced cyclobutane pyrimidine dimers.
• Multiple repair mechanisms exist in microbes and eukaryotes.
• Error tolerance varies by pathway.

Concept / Approach:
Photoreactivation (photolyase mediated) directly reverses dimers using visible light and is considered error free because it restores the original bases. Nucleotide excision repair removes a short oligonucleotide containing the lesion and fills the gap using the opposite strand as a template, typically maintaining fidelity. SOS repair, in contrast, induces specialized translesion synthesis polymerases that can replicate across lesions but lack high fidelity, raising the chance of base substitutions or small insertions and deletions.

Step-by-Step Solution:
List candidate mechanisms: photoreactivation, excision repair, SOS repair. Classify their fidelity: photoreactivation and excision are largely accurate. Recognize SOS as an inducible, damage tolerant, error prone system. Select SOS repair as most likely to introduce mutations.

Verification / Alternative check:
Mutator phenotypes with elevated translesion polymerase activity show increased spontaneous mutation rates, validating the error prone nature of SOS responses.

Why Other Options Are Wrong:

• Excision repair: template directed gap filling preserves sequence.
• Photoreactivation: direct lesion reversal without sequence change.
• Never leads to mutation: unrealistic, given SOS polymerase properties.

Common Pitfalls:
Assuming all repair improves fidelity; some pathways prioritize survival over accuracy when damage is extensive.

Final Answer:
SOS repair