The RNA sequence below depicts the part of a 330 nucleotides long mRNA and it encodes the C-terminal portion of a protein.
5'-… … … AAC ACC ACG ACC CAU GUG GCG AGA CGG UAG-3'
A mutation was identified in this RNA denoted as 322A→U. This nucleotide change is represented by which of the following class(es) of mutation?
Missense mutation
Non-sense mutation
Transversion mutation
Silent mutation
ANSWER
Codon 322-324 (AGA, Arg) is mutated by 322A→U to UGA, a stop codon, causing premature chain termination before the natural UAG stop — this is a nonsense mutation (B). Independently, A (purine) → U (pyrimidine) is a purine-to-pyrimidine substitution, i.e. a transversion (C). So the correct answer is B and C.
Official answer: B, CSource: Official answer key, GATE 2026 BT (IIT Guwahati)
The 30-nt printed sequence is 10 codons = the LAST 30 nt of the 330-nt mRNA, i.e. positions 301-330 (reading frame starting at nt 1).
Splitting into codons: codon8 (positions 322-324) = AGA, codon9 (325-327) = CGG, codon10 (328-330) = UAG.
AGA and CGG both code for Arginine (Arg); UAG is the natural stop codon that ends this protein's C-terminus.
Apply the mutation 322A→U: codon8 AGA becomes UGA.
UGA is one of the three stop codons (UAA, UAG, UGA) — translation now halts at codon8 instead of continuing through Arg(CGG) to the natural UAG stop.
A sense codon (Arg) turned into a stop codon, causing premature chain termination, is by definition a nonsense mutation → option (B) correct. It is not missense (no substituted amino acid — translation stops) and not silent (product is truncated, not identical) → (A) and (D) are wrong.
Classify the base substitution itself, independent of its coding effect: A is a purine (adenine), U is a pyrimidine (uracil).
A purine→pyrimidine substitution is, by definition, a transversion (as opposed to a transition, which is purine↔purine or pyrimidine↔pyrimidine) → option (C) correct.
Since the question is MSQ (one or more correct) and both the coding-consequence classification and the base-chemistry classification describe the same single mutational event, the final answer is (B) and (C).
KEY IDEA
Classify a point mutation on two independent axes at once: (1) its effect on the encoded amino acid/reading (silent / missense / nonsense) by translating the codon before and after the change, and (2) the chemical nature of the base substitution itself (transition: purine↔purine or pyrimidine↔pyrimidine, vs transversion: purine↔pyrimidine) — an MSQ can require both classifications to be marked correct simultaneously.
WATCH OUT FOR
Forgetting that the printed 30-nt segment is the LAST 10 codons of a 330-nt (110-codon) message and mis-numbering which codon contains position 322 — or picking only the coding-consequence answer (nonsense, B) while overlooking that the question also independently asks about the transversion/transition nature of the base change (C), missing the second correct option in this MSQ.
From the GATE 2026 BT paper. Every question of this paper · Download the full paper · All 9 Mutation & repair questions, with notes
