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 $322A \rightarrow U$. This nucleotide change is represented by which of the following class(es) of mutation?

Question

Which of the following bonds is responsible for maintaining the secondary structure of proteins?

Answer 1

This question checks two things at once: whether you can find a specific nucleotide inside a partial mRNA sequence using its stated total length, and whether you know how to classify a point mutation both by its effect on the protein and by its base chemistry.

The full transcript is 330 nucleotides long. The piece shown is the tail end of it, so its last base is nucleotide 330. Splitting the shown sequence into triplets counting back from the 3' end: the stop codon $UAG$ sits at positions 328-330, the codon before it, $CGG$, sits at 325-327, and the one before that, $AGA$, sits at 322-324. Nucleotide 322 is therefore the A that starts the codon $AGA$.

Before the mutation, $AGA$ codes for arginine. The change is $322A \to U$, so the codon reads $UGA$ afterward, and $UGA$ is a stop codon (one of $UAA$, $UAG$, $UGA$). Turning an amino-acid codon into a stop codon cuts the protein short partway through, which is exactly what a nonsense mutation means.

Missense mutation: wrong, the new codon does not code for a different amino acid, it codes for a stop signal instead.
Non-sense mutation: correct, a codon for an amino acid (Arg) has become a stop codon.
Transversion mutation: correct, checked separately below by base chemistry.
Silent mutation: wrong, the amino acid at this position does change, from arginine to a stop, so the protein sequence itself is altered.

For the chemistry: bases split into purines ($A$, $G$, two-ring structures) and pyrimidines ($C$, $U$, one-ring structures). A substitution that stays inside one group is a transition. A substitution that jumps from one group to the other is a transversion. Here $A$ (purine) is replaced by $U$ (pyrimidine), so the change crosses groups and is a transversion.

Let's summarize:

Position 322 falls in the codon $AGA$ (Arg), the eighth codon counted back from the stop codon of this 330-nt transcript.
$AGA \to UGA$ swaps an arginine codon for a stop codon: a nonsense mutation.
$A \to U$ swaps a purine for a pyrimidine: a transversion.

So both (B) Non-sense mutation and (C) Transversion mutation correctly describe this change.

Answer 2

This question checks two things at once: whether you can find a specific nucleotide inside a partial mRNA sequence using its stated total length, and whether you know how to classify a point mutation both by its effect on the protein and by its base chemistry.

The full transcript is 330 nucleotides long. The piece shown is the tail end of it, so its last base is nucleotide 330. Splitting the shown sequence into triplets counting back from the 3' end: the stop codon $UAG$ sits at positions 328-330, the codon before it, $CGG$, sits at 325-327, and the one before that, $AGA$, sits at 322-324. Nucleotide 322 is therefore the A that starts the codon $AGA$.

Before the mutation, $AGA$ codes for arginine. The change is $322A \to U$, so the codon reads $UGA$ afterward, and $UGA$ is a stop codon (one of $UAA$, $UAG$, $UGA$). Turning an amino-acid codon into a stop codon cuts the protein short partway through, which is exactly what a nonsense mutation means.

Missense mutation: wrong, the new codon does not code for a different amino acid, it codes for a stop signal instead.
Non-sense mutation: correct, a codon for an amino acid (Arg) has become a stop codon.
Transversion mutation: correct, checked separately below by base chemistry.
Silent mutation: wrong, the amino acid at this position does change, from arginine to a stop, so the protein sequence itself is altered.

For the chemistry: bases split into purines ($A$, $G$, two-ring structures) and pyrimidines ($C$, $U$, one-ring structures). A substitution that stays inside one group is a transition. A substitution that jumps from one group to the other is a transversion. Here $A$ (purine) is replaced by $U$ (pyrimidine), so the change crosses groups and is a transversion.

Let's summarize:

Position 322 falls in the codon $AGA$ (Arg), the eighth codon counted back from the stop codon of this 330-nt transcript.
$AGA \to UGA$ swaps an arginine codon for a stop codon: a nonsense mutation.
$A \to U$ swaps a purine for a pyrimidine: a transversion.

So both (B) Non-sense mutation and (C) Transversion mutation correctly describe this change.

Show Hint

The Correct Option is B, C

Solution and Explanation

Questions Asked in GATE BT exam