Q.67 Which one of the following mutagens convert DNA’s adenine to hypoxanthine?
(A) Ultraviolet light
(B) Mitomycin C
(C) Methyl methanesulfonate
(D) Nitrous acid
Nitrous acid (option D) is the mutagen that converts DNA’s adenine to hypoxanthine through deamination. This specific reaction leads to transition mutations during replication, as hypoxanthine pairs with cytosine instead of thymine.
Option Analysis
Ultraviolet light (A) causes DNA damage primarily via cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts between adjacent pyrimidines, not base deamination.
Mitomycin C (B) acts as a DNA crosslinker after reductive activation, alkylating guanine and adenine at specific sites like N6 of adenine and O6/N2 of guanine, without converting adenine to hypoxanthine.
Methyl methanesulfonate (C) alkylates DNA bases, mainly at N7-guanine and N3-adenine, leading to apurinic sites and replication fork stalling, but does not deaminate adenine.
Nitrous acid (D) deaminates adenine (removing the amino group) to form hypoxanthine, which base-pairs with cytosine, causing A-T to G-C transitions.
The mutagen that converts DNA adenine to hypoxanthine is nitrous acid, a key topic in CSIR NET Life Sciences for understanding DNA mutagenesis. This deamination reaction alters base pairing and drives transition mutations, making it essential for competitive exam preparation in molecular biology.
Mutagen Mechanisms
Nitrous acid (HNO₂) removes the amino group (-NH₂) from adenine, yielding hypoxanthine, which forms two hydrogen bonds with cytosine instead of thymine’s three with adenine. During replication, this shifts A-T pairs to G-C, producing AT→GC transitions. Other bases like cytosine (to uracil) and guanine (to xanthine) are also affected, but adenine-to-hypoxanthine is specific here.
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UV light induces pyrimidine dimers, repaired by nucleotide excision repair.
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Mitomycin C crosslinks DNA at CpG sites post-activation.
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MMS adds methyl groups, creating lethal adducts.
CSIR NET Relevance
Questions like “Which one of the following mutagens convert DNA’s adenine to hypoxanthine?” test deamination vs. alkylation or photoproducts. Nitrous acid mutations are reversible by the same agent, aiding exam recall.
Biological Impact
Hypoxanthine lesions trigger base excision repair via endonuclease V or AlkA glycosylases; unrepaired, they cause chronic inflammation-linked cancers. This underscores mutagenesis in evolution and disease for CSIR NET syllabi.
