Molecular Biology & Genetics · NEET PG Preparation Biochemistry
A 4-year-old child presents with extreme photosensitivity, dry skin, and multiple pigmented spots on sun-exposed areas. A skin biopsy fibroblast cult…
Practice question from Molecular Biology & Genetics (NEET PG Preparation Biochemistry), with the correct answer and a step-by-step solution.
medium · MCQ · Q-668B77
A 4-year-old child presents with extreme photosensitivity, dry skin, and multiple pigmented spots on sun-exposed areas. A skin biopsy fibroblast culture reveals a defect in nucleotide excision repair (NER). This enzymatic defect most directly impairs the removal of which of the following?
- A.Pyrimidine dimers induced by ultraviolet lightCorrect answer
- B.O6-methylguanine adducts caused by alkylating agents
- C.Double-strand breaks caused by ionizing radiation
- D.Mismatched bases due to replication slippage
Solution
The patient has Xeroderma Pigmentosum, characterized by a defect in nucleotide excision repair (NER). NER is responsible for removing bulky DNA lesions such as pyrimidine dimers (cyclobutane pyrimidine dimers and 6-4 photoproducts) caused by UV light. O6-methylguanine is repaired by direct reversal (methyltransferase). Double-strand breaks are repaired by non-homologous end joining or homologous recombination. Mismatches are repaired by mismatch repair (MMR).
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