What is the result of a nonsense mutation in a coding sequence?

Question

BlackTom AI · Accepted Answer

When considering the effect of a nonsense mutation in a coding sequence, we evaluate what a premature stop codon does to the protein product.
Option 1: 'A mutation that does not change amino acid sequence, but alters mRNA stability' describes a synonymous or small regulatory change rather than a nonsense mutation. It does not introduce a stop codon, so it would not terminate translation early.
Option 2: 'Premature stop codon' correctly identifies the hallmark of a nonsense mutation: a codon that signals termination of translation earlier than normal, yielding a truncated protein.
Option 3: 'Repeat expansion' refers to a gain-of-function or disease mechanism where short DNA repeats increase in number, which is distinct from a nonsense mutation and does not inherently create a premature stop codon.
Option 4: 'A change in the reading frame' denotes a frameshift mutation, which alters downstream codons and often creates a premature stop later, but it is not the same as a nonsense mutation per se; a frameshift can produce new stop codons but the primary characteristic described here is a direct stop codon introduced without frameshifting.
In summary, the nonsense mutation directly yields an early stop codon, causing premature termination of translation and a truncated protein, whereas the other options describe different mutation classes or consequences that do not match the definition of a nonsense mutation.

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