You have the following mutation in the DNA:

What caused this mutation and how would you fix it?

Question

You have the following mutation in the DNA:

What caused this mutation and how would you fix it?

BlackTom AI · Accepted Answer

Adjacent thymine bases joined by a covalent link (a kinked backbone with two T’s bonded) are characteristic of a UV‑induced thymine (pyrimidine) dimer; this lesion is specifically repaired by photoreactivation using photolyase.

1) Relevant concepts
- UV light (particularly UV-B/UVC) induces covalent bonding between adjacent pyrimidines (most often two thymines), producing cyclobutane pyrimidine dimers that distort the DNA helix.  
- Photoreactivation (photolyase) is an enzyme-mediated repair that directly cleaves the dimer bond using energy from visible/blue light, restoring the original bases.  
- Other repair systems (mismatch repair, SOS, recombinational repair, nucleotide excision) address different lesion types (replication mismatches, extensive damage/double-strand breaks, bulky adducts) and are not the primary fix for simple thymine dimers.

2) Step-by-step reasoning
- Inspect the drawing: two adjacent T residues on the same strand are shown joined (yellow marks) and the helix is locally kinked → identifies a thymine dimer.  
- Known cause: thymine dimers form when UV photons induce covalent bonds between adjacent pyrimidines → therefore the mutagen is UV light.  
- Known repair: photolyase binds the dimer and, upon absorbing visible light, breaks the cyclobutane ring, restoring unmodified thymines → photoreactivation is the appropriate fix.

3) (No numeric calculations needed; the “intermediate steps” are the lesion identification and mapping to cause/repair above.)

4) Why the correct answer fits
- The shown lesion equals a thymine dimer caused by UV; photoreactivation specifically reverses thymine dimers in organisms that have photolyase.

5) Why other choices are incorrect
- Replication error / SOS repair: replication errors produce mismatched bases, not covalently linked adjacent thymines; SOS is a broad damage response, not the specific direct reversal.  
- Chemical mutagen / recombination repair: chemical mutagens usually cause base modifications or adducts; recombination repair addresses double‑strand problems.  
- UV light / methyl‑mismatch repair: although UV is correct here, methyl‑mismatch (mismatch repair) targets replication mismatches, not thymine dimers.

Conclusion: The lesion is caused by UV light and would be fixed by photoreactivation (photolyase), matching the provided answer.

You have the following mutation in the DNA: What caused this mutation and how would you fix it?单项选择题

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