Question

Which DNA repair mechanism corrects UV-induced thymine dimers?

• A. Base excision repair
• B. Nucleotide excision repair
• C. Mismatch repair
• D. Photoreactivation only

Hint:

Associate \textbf{N}ucleotide \textbf{E}xcision \textbf{R}epair with bulky, "large scale" damage that distorts the double helix (like UV dimers). Associate \textbf{B}ase \textbf{E}xcision \textbf{R}epair with small, specific base alterations.

Answer 1

The Correct Option is B

The question asks which DNA repair mechanism is responsible for correcting UV-induced thymine dimers. Let's analyze the options and identify the correct mechanism.

1. Base excision repair (BER): This mechanism typically addresses small, non-helical-distorting base lesions, such as single-base mismatches, oxidized bases, or small non-bulky adducts. Therefore, it is not equipped to handle thymine dimers, which cause distortions in the DNA helix.
2. Nucleotide excision repair (NER): This DNA repair mechanism is specially designed to remove bulky, helix-distorting lesions like thymine dimers caused by UV light. The NER process involves the recognition of the DNA lesion, excision of a short single-stranded DNA segment containing the thymine dimers, and synthesis of the correct DNA strand using the undamaged strand as a template.
3. Mismatch repair (MMR): This repair system corrects errors that escape proofreading during DNA replication, such as base-pair mismatches and small insertion-deletion loops. MMR is not involved in repairing thymine dimers.
4. Photoreactivation: This is a direct repair mechanism primarily found in some bacteria, fungi, and plants. It involves photolyase enzymes that use light to split the thymine dimer back into two separate thymines. However, in humans, nucleotide excision repair is the prominent mechanism for repairing these lesions instead of photoreactivation.

From the analysis, it is clear that nucleotide excision repair is the correct mechanism for correcting UV-induced thymine dimers.

In conclusion, the option Nucleotide excision repair is the correct answer.