Q.79 Which of the following is/are consequence(s) of nitrous acid (HNO2) mediated
deamination?
(A) Deamination of cytosine, adenine and guanine
(B) GC–to–AT transitions
(C) AT– to– GC transitions
(D) Addition of alkyl group to the bases
Nitrous acid (HNO₂) primarily causes deamination of DNA bases with amino groups, leading to specific transition mutations relevant for CSIR NET Life Sciences preparation. This mutagen affects adenine, cytosine, and guanine, producing both GC-to-AT and AT-to-GC transitions, but does not involve alkylation.
Option Analysis
Nitrous acid deaminates bases containing exocyclic amino groups (-NH₂). Cytosine converts to uracil (pairs with A instead of G), adenine to hypoxanthine (pairs with C instead of T), and guanine to xanthine (can pair with C or T).
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(A) Deamination of cytosine, adenine and guanine: Correct. These three bases lose their amino groups via HNO₂-mediated oxidation.
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(B) GC-to-AT transitions: Correct. Cytosine→uracil (C:G → T:A) or guanine→xanthine (G:C → A:T after replication).
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(C) AT-to-GC transitions: Correct. Adenine→hypoxanthine (A:T → G:C).
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(D) Addition of alkyl group to the bases: Incorrect. Alkylation involves agents like EMS or nitrosamines adding alkyl groups (e.g., methyl); HNO₂ acts via deamination.
Correct answer: A, B, C.
Nitrous acid deamination consequences dominate CSIR NET Life Sciences questions on DNA mutagenesis. This chemical mutagen targets amino groups in DNA bases, altering base-pairing and causing transition mutations critical for exam preparation.
Deamination Mechanism
HNO₂ reacts with primary amines on bases, forming diazonium ions that lose N₂, yielding keto forms. Cytosine → uracil (U:G mismatch → T:A), adenine → hypoxanthine (pairs with C), guanine → xanthine (G:C mismatch). Thymine lacks an amino group, remaining unaffected.
Mutation Types Induced
Both transition mutations occur:
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GC → AT: From C → U or G → X.
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AT → GC: From A → hypoxanthine.
No transversions or frameshifts; exclusively transitions.
| Mutation | Original Pair | Deaminated Base | Resulting Pair | Example Agent Comparison |
|---|---|---|---|---|
| GC → AT | G:C | C → U | G:T (→ A:T) | HNO₂ (deamination) |
| AT → GC | A:T | A → Hypoxanthine | Hypoxanthine:C (→ G:C) | HNO₂ |
| GC → AT | G:C | G → Xanthine | Xanthine:T (→ A:T) | HNO₂ |
CSIR NET Relevance
Questions test base specificity (A correct) and mutations (B, C correct). D is wrong—alkylation by EMS/MMS. Repair involves base excision (e.g., uracil-DNA glycosylase for U). Practice distinguishes deaminating (HNO₂, bisulfite) from alkylating mutagens.
