Nucleotide excision repair and photolyases would be beneficial for bacterial strains _________________.

Question

BlackTom AI · Accepted Answer

Photolyases directly reverse ultraviolet-induced cyclobutane pyrimidine dimers (CPDs), and nucleotide excision repair (NER) removes bulky, helix-distorting lesions such as those same UV-induced pyrimidine dimers—therefore both systems are especially useful for bacteria exposed to UV light.

1) Relevant knowledge points and concepts
- UV radiation (particularly UV-B/UVC) induces covalent bonds between adjacent pyrimidines (most commonly thymine–thymine), forming cyclobutane pyrimidine dimers and 6-4 photoproducts that distort the DNA helix and block replication/transcription.
- Photolyases perform light-dependent “photoreactivation”: an enzyme binds the dimer and, upon absorbing blue/near-UV light with its chromophore and reduced FAD cofactor, transfers an electron to split the dimer, restoring two normal pyrimidines.
- Nucleotide excision repair is a multienzyme pathway that recognizes bulky helix-distorting lesions, excises an oligonucleotide segment containing the lesion, and fills/seals the gap.

2) Step-by-step reasoning process
- UV exposure → formation of pyrimidine dimers that impede DNA function.
- Photolyase mechanism: enzyme binds dimer → absorbs light → electron transfer breaks cyclobutane ring → lesion reversed without cutting backbone.
- NER mechanism in bacteria (Uvr system): UvrA+UvrB detect distortion → UvrC makes dual incisions flanking lesion → UvrD helicase removes the oligonucleotide fragment → DNA polymerase I synthesizes replacement nucleotides → DNA ligase seals nick.
- Both mechanisms therefore prevent mutations and restore normal DNA after UV damage.

3) Intermediate steps shown (NER enzymes): recognition (UvrA/UvrB) → incision (UvrC) → excision (UvrD) → resynthesis (DNA pol I) → ligation.

4) Why the correct answer is correct
Both photolyases and NER specifically target and repair the bulky, covalent pyrimidine dimers produced by UV light. Photolyase provides a rapid light-dependent reversal; NER provides a light-independent excision and replacement pathway—complementary defenses against UV damage.

5) Why other options are incorrect
- “With oxidized guanine (G) bases”: oxidized bases like 8-oxoG are primarily repaired by base excision repair (glycosylases), not photolyase/NER.
- “With extensive DNA strand breaks”: strand breaks are fixed by recombinational repair or end-joining processes and require different enzymes (e.g., RecA-mediated homologous recombination), not photolyase.
- “Lacking DNA polymerase”: absence of DNA polymerase prevents gap filling after excision; photolyase also requires light and cannot substitute for polymerase functions—these repair systems do not correct the fundamental problem of missing polymerase activity.

Conclusion: Photolyases and NER are most beneficial for bacterial strains exposed to UV light; this matches the provided correct answer: exposed to UV light.

Nucleotide excision repair and photolyases would be beneficial for bacterial strains _________________. Single choice

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