A cell from a patient with Xeroderma pigmentosum is irradiated with UV light. The predominant DNA lesion formed and the specific repair mechanism that is defective in this disease are:

• A Single-strand breaks; base excision repair (BER)
• B 8-Oxoguanine adducts; mismatch repair (MMR)
• C Double-strand breaks; homologous recombination (HR)
• D Cyclobutane pyrimidine dimers (CPD); nucleotide excision repair (NER) ✓

Explanation

UV radiation generates cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts between adjacent pyrimidines (primarily T-T), which distort the DNA helix. Nucleotide excision repair (NER) is the pathway responsible for recognizing and removing bulky helix-distorting lesions; NER involves ~30 proteins including XPA, XPC, TFIIH, XPG, and XPF-ERCC1. In xeroderma pigmentosum, any one of the XP proteins (XPA–XPG) may be mutated, leading to UV hypersensitivity and skin cancers. Mismatch repair corrects replication errors.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.