Question

what is the effect of a nucleotide-pair substitution that results in a nonsense mutation in a gene?
○ it changes an amino acid in the encoded protein.
○ it has no effect on the amino acid sequence of the encoded protein.
○ it introduces a premature stop codon into the mrna.
○ it alters the reading frame of the mrna.

Explanation:

1. Analyze the first option: A missense mutation changes an amino acid, not a nonsense mutation. So this is incorrect.
2. Analyze the second option: A silent mutation has no effect on the amino acid sequence, not a nonsense mutation. So this is incorrect.
3. Analyze the third option: By definition, a nonsense mutation is a type of mutation in DNA or RNA that results in a premature stop codon in the mRNA, which leads to the termination of translation earlier than normal. So this is correct.
4. Analyze the fourth option: Frameshift mutations (like insertions or deletions of non - multiple of 3 nucleotides) alter the reading frame, not nonsense mutations. So this is incorrect.

Answer:

C. It introduces a premature stop codon into the mRNA.