DNA damage and direct repair Ultraviolet (UV)–induced thymine dimers in DNA can be directly reversed with the help of visible light by which repair process?

Introduction / Context:
UV radiation creates cyclobutane pyrimidine dimers (most commonly thymine dimers) that distort the DNA helix and block polymerases. Cells deploy several repair pathways; among them, photoreactivation is a unique, light-dependent direct reversal mechanism mediated by photolyase enzymes.

Given Data / Assumptions:

• Damage type: UV-induced thymine dimers.
• Some organisms possess photolyase; others (including placental mammals) do not.
• Visible light is required to activate photolyase for direct reversal.

Concept / Approach:
Photoreactivation uses a DNA photolyase that absorbs visible light and uses that energy to cleave the cyclobutane ring linking adjacent thymines, restoring normal base pairing without excising nucleotides. This contrasts with nucleotide excision repair, which removes a short stretch of damaged DNA and fills the gap by synthesis.

Step-by-Step Solution:
Identify damage: thymine dimers caused by UV.Recall direct reversal pathway: photoreactivation using photolyase.Note requirement: visible light provides energy for the reaction.Select “Photoreactivation.”

Verification / Alternative check:
Genetic studies show organisms lacking photolyase rely more heavily on excision repair and exhibit higher UV sensitivity in the absence of light-mediated repair.

Why Other Options Are Wrong:

• Phosphorylation: protein modification, not DNA lesion reversal.
• Excision repair: removes and resynthesizes rather than directly reversing the dimer.
• Photosynthesis: energy-harvesting process, unrelated to DNA repair.
• Translesion synthesis: bypasses lesions, does not repair them.

Common Pitfalls:
Assuming all UV repair is excision-based; photoreactivation is a distinct light-driven pathway when photolyase is present.

Final Answer:
Photoreactivation.