Genetic Code

Consider a gene that is mutated, generating the change in the mRNA shown below. (Assume that this is a piece of mRNA from within the middle of the protein-coding sequence.)

mRNA from original gene: 5’-CCUGGCUAUUGC-3’
mRNA from mutated gene: 5’-CCUGCGCUAUUGC-3’

What type of mutation is this?

Question

Genetic Code

Consider a gene that is mutated, generating the change in the mRNA shown below. (Assume that this is a piece of mRNA from within the middle of the protein-coding sequence.)

mRNA from original gene:              5’-CCUGGCUAUUGC-3’
mRNA from mutated gene:            5’-CCUGCGCUAUUGC-3’

What type of mutation is this?

BlackTom AI · Accepted Answer

To begin, let's lay out the given information and compare the original and mutated mRNA sequences carefully.
Original mRNA: 5’-CCUGGCUAUUGC-3’
Mutated mRNA:  5’-CCUGCGCUAUUGC-3’
When we segment these into codons from the 5’ end, the original sequence reads as: CCU | GGC | UAU | UGC.
The mutated sequence reads as: CCU | GCG | CUA | UUG.
Next, observe what changed between the two sequences. The first and third codons (CCU and UAU in the original; CCU and CUA in the mutated) are not identical, but the key difference is in the second codon: GGC (original) changed to GCG (mutated).
Now, assess the amino acid impact:
- GGC codes for Glycine, while GCG codes for Alanine. Therefore, this single-nucleotide substitution within the second codon changes the encoded amino acid, altering the protein sequence at that position. This fits the definition of a missense mutation, where one amino acid is replaced by another due to a nucleotide change.
Consider the other possible mutation types and why they do not apply here:
- A silent mutation would occur if the codon change did not alter the amino acid sequence (same amino acid despite the nucleotide change). Since GGC and GCG code for different amino acids, this is not silent.
- A nonsense mutation would introduce a stop codon (UAA, UAG, UGA). Neither GGC nor GCG becomes a stop codon, so this is not a nonsense mutation.
- A deletion mutation involves removing one or more nucleotides, changing the reading frame or shortening the sequence. Here, the length remains the same and only a single base changed, not deleted, so it is not a deletion.
Putting it together, the evidence points to a single-base substitution that changes one amino acid to another, i.e., a missense mutation. The other options fail to capture the observed amino acid change or rely on different types of sequence alterations.
Thus, option: A missense mutation correctly describes the mutation type in this scenario.

BIOL 1009 (001-006/050-055) Midterm Exam III

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