Question textConsider the codon table below for this question.Below is the Wildtype version (WT) of the coding region of an mRNA including a portion of the 3’ untranslated region (UTR), with its resulting amino acid sequence above it. You have discovered a mutant version (M) of the gene. Met Thr Ser Arg Thr TyrWT 5’-GGC CCA AUG ACG UCA AGA ACC UAU UAA GAU… 3’ UTR continuesM 5’-GGC CCA AUG ACG UAA AGA ACC UAU UAA GAU… 3’ UTR continuesa) What change has occurred in the mutant sequence? Answer 1 Question 9[select: , Nucleotide substitution(s), Nucleotide insertion(s), Nucleotide deletion(s), Nucleotide fragmentation(s)]b) What type of mutation has this caused? Answer 2 Question 9[select: , Frameshift mutation, Missense mutation, In-frame deletion, Silent mutation, In-frame insertion, Nonsense mutation]

Question

BlackTom AI · Accepted Answer

To tackle this question, I’ll compare the wildtype (WT) coding sequence with the mutant (M) sequence and examine how a single change affects the codons and the resulting protein.
Option a) Nucleotide substitution(s) vs the other possibilities:
- In the WT sequence, the codon at the fifth position is UCA (which encodes Ser). In the mutant, that same region is UAA (which encodes a Stop). This is a single-nucleotide change within the codon (UCA → UAA), not an insertion or a deletion of nucleotides, and there is no fragmentation of the sequence. Therefore, the alteration is best described as a nucleotide substitution(s).
- If it were an insertion, deletion, or fragmentation, you would expect a shift in the reading frame or a partial shift in codon boundaries, which is not indicated here since the rest of the sequence remains in-frame with the same codon boundaries after the change. Hence, the other options (insertion, deletion, fragmentation) do not fit.
Option b) Types of mutation (Frame shift, Missense, In-frame deletion, Silent, In-frame insertion, Nonsense):
- Frameshift mutation would require an insertion or deletion that shifts the reading frame, altering downstream codons. Here, only one codon changed from UCA to UAA without changing the downstream frame, so a frameshift is not correct.
- Missense mutation is when a codon change results in a different amino acid; here, the codon changes to UAA, which is a Stop codon, so it is not a missense mutation.
- In-frame deletion or insertion would alter amino acids within the same frame without introducing a Stop codon in the early region; the presence of a Stop codon at this position argues against in-frame indels.
- Silent mutation would mean the codon change does not alter the amino acid; since UCA encodes Ser and UAA encodes Stop, the amino acid sequence is affected (and is truncated by a Stop), so it cannot be silent.
- Nonsense mutation is defined as a single-nucleotide change that converts a sense codon into a Stop codon, resulting in premature termination of translation. This precisely describes the observed change from Ser (UCA) to Stop (UAA) at that position.

In summary, the observed change is a nucleotide substitution that converts a sense codon to a Stop codon, i.e., a nonsense mutation.

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