Xeroderma pigmentosum (XP) is caused by defective nucleotide excision repair (NER). The specific type of DNA damage that NER corrects, and which is caused by UV radiation, is:

• A Oxidative base damage (8-oxoguanine)
• B Alkylation adducts on adenine (N3-methyladenine)
• C Cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts ✓
• D Double-strand breaks at telomeres

Explanation

UV-B radiation induces the formation of covalent bonds between adjacent pyrimidines on the same strand: primarily cyclobutane pyrimidine dimers (CPDs, thymine-thymine most common) and 6-4 photoproducts. These bulky adducts distort the double helix and are recognised and removed by nucleotide excision repair. XP patients lack one of several XPC, XPA, TFIIH, or other NER proteins, leaving UV-induced dimers unrepaired; this causes mutational hotspots in TP53 and other genes, leading to multiple skin malignancies at sun-exposed areas. Oxidative damage is repaired by base excision repair (BER); alkylation by BER/direct repair.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.