In a gene that normally contains three exons, which of the following changes probably will NOT activate the nonsense-mediated mRNA decay pathway? The sizes of exons 1 to 3 are 100, 150, and 200 nucleotide pairs, respectively.
A) A nonsense mutation in exon 1.
B) A nonsense mutation in exon 2.
C) A mutation in the first intron resulting in the inclusion of a large intronic fragment in the mature mRNA.
D) A frameshift mutation in exon 1.
E) A mutation in exon 2 leading to its loss through exon skipping.

Question

Quizplus · Accepted Answer

Nonsense mutations in the upstream exons can directly trigger nonsense-mediated decay (NMD). Frameshift mutations in these exons are likely to introduce premature termination codons and activate NMD as well. Similarly, retention of intronic sequences carries a high risk of premature stop codons. In some cases, exon skipping can lead to NMD if the size of the skipped exon is not a multiple of 3 and therefore its absence changes the reading frame of the downstream exon.

In a Gene That Normally Contains Three Exons, Which of the Following