Correct Answer: Photolyase (Option 4)                                                                                                                                                                                                                                                                                When DNA is exposed to ultraviolet (UV) radiation, it suffers damage primarily in the form of cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts. These lesions distort the DNA structure and can cause mutations if not repaired. Among the various DNA repair mechanisms, direct repair is a process that reverses the damage without removing the bases or nucleotides, thus restoring the original DNA structure efficiently.

The enzyme responsible for this direct repair mechanism against UV-induced DNA damage is photolyase. Photolyase specifically binds to UV-induced lesions such as CPDs and uses energy from visible light (blue or near-UV light) to break the abnormal bonds formed by UV radiation, restoring the DNA to its undamaged state145.

How Photolyase Works

• Photolyase recognizes and binds tightly to the damaged DNA site.

• It absorbs light energy, usually in the blue or UVA range.

• This energy activates the enzyme to transfer an electron to the damaged site.

• The electron transfer breaks the cyclobutane ring of the dimer, reversing the lesion.

• The DNA is restored to its original form, and photolyase dissociates from the DNA15.

This repair process, known as photoreactivation, is a highly efficient and rapid method to directly reverse UV-induced DNA damage without excising the damaged bases.

Why Other Options Are Not Correct

• RNA polymerase (Option 1): While RNA polymerases can detect DNA lesions during transcription and help initiate repair mechanisms, they do not directly repair UV-induced DNA damage. Instead, they stall at lesions and recruit repair proteins68.

• Uvr ABC (Option 2): The UvrABC endonuclease complex is involved in nucleotide excision repair (NER), an indirect repair mechanism that removes damaged DNA segments and fills in the gap. It is not a direct repair enzyme.

• Restriction endonuclease (Option 3): These enzymes cut DNA at specific sequences but are not involved in repairing UV-induced DNA damage.

Importance of Photolyase in DNA Repair

Photolyase is present in plants, bacteria, and some animals but is notably absent in humans and other mammals. This absence explains why humans rely on other repair pathways, like nucleotide excision repair, to fix UV damage. Photolyase’s ability to directly reverse UV damage is thought to contribute to the higher UV resistance observed in plants and some microorganisms5.

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