Base excision, nucleotide excision and mismatch repair are strategies the cell uses to correct certain DNA mutations. Which of the following enables these repair mechanisms to occur accurately?  

Question

Base excision, nucleotide excision and mismatch repair are strategies the cell uses to correct certain DNA mutations. Which of the following enables these repair mechanisms to occur accurately?

BlackTom AI · Accepted Answer

Because DNA is double-stranded, the undamaged complementary strand serves as an accurate template for restoring information lost or altered on one strand. This central feature underlies base excision repair (BER), nucleotide excision repair (NER) and mismatch repair (MMR).

1) Relevant concepts:
- BER removes a single damaged base (glycosylase → AP site), then AP endonuclease and phosphodiesterases remove the backbone, creating a short gap that is filled by a DNA polymerase using the opposite strand as template and sealed by ligase.
- NER excises an oligonucleotide segment containing bulky lesions, leaving a gap that DNA polymerase fills by copying the complementary strand, followed by ligation.
- MMR recognizes base–base mismatches or small insertion/deletion loops after replication, excises the error-containing segment on the newly synthesized strand, and resynthesizes DNA using the parental (correct) strand as template.
- Strand discrimination mechanisms (e.g., transient lack of methylation on daughter strands in bacteria or nicks/replication signals in eukaryotes) guide which strand is replaced.

2) Step-by-step reasoning (example for BER):
- Damaged base is removed by specific glycosylase → AP site.
- AP endonuclease cleaves backbone → gap.
- DNA polymerase (e.g., Pol I in bacteria, Pol β/δ/ε in eukaryotes) inserts correct nucleotide(s) using the intact complementary strand sequence.
- DNA ligase seals the remaining nick.

3) Intermediate steps are shown above for BER; NER and MMR follow analogous remove-gap-fill-seal sequences with the intact complementary strand as template.

4) Why this is correct: Each pathway relies on copying the correct sequence from the opposing strand to restore the original base order. Without a complementary template, accurate repair would be impossible.

5) Why other options are incorrect:
- Chromosome pairing/recombination: mainly for double-strand break repair and meiotic exchange, not the routine BER/NER/MMR mechanisms.
- DNA ligase: only seals nicks; it cannot determine the correct base to insert.
- RNA polymerase: transcription-coupled repair uses stalled RNA pol to recruit repair factors, but RNA is not used as a template to replace DNA bases.

Conclusion: The correct answer is "DNA is double stranded, and the complementary strand can be used as a template for repair," which matches the provided answer.

Base excision, nucleotide excision and mismatch repair are strategies the cell uses to correct certain DNA mutations. Which of the following enables these repair mechanisms to occur accurately?  单项选择题

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Base excision, nucleotide excision and mismatch repair are strategies the cell uses to correct certain DNA mutations. Which of the following enables these repair mechanisms to occur accurately? 单项选择题