A gene encodes a pre-mRNA with 5 exons and 4 introns. A mutation eliminates the 3’ splice site from intron 3. What exons would be found in the mRNA after splicing was completed?

Question

A gene encodes a pre-mRNA with 5 exons and 4 introns.  A mutation eliminates the 3’ splice site from intron 3. What exons would be found in the mRNA after splicing was completed?

BlackTom AI · Accepted Answer

Splicing depends on paired 5' (donor) and 3' (acceptor) splice sites flanking introns; the spliceosome normally removes each intron by joining the upstream exon’s 3' end to the downstream exon’s 5' end. In a pre-mRNA with exon1–intron1–exon2–intron2–exon3–intron3–exon4–intron4–exon5, loss of the 3' splice (acceptor) site for intron 3 means the acceptor immediately upstream of exon 4 cannot be recognized.

Step-by-step reasoning:
1. Introns 1 and 2 are removed normally using their intact 5' and 3' splice sites, joining exon1–exon2–exon3.
2. For intron 3, the 5' donor site (at the exon3/intron3 boundary) is present but the corresponding 3' acceptor (at the intron3/exon4 boundary) is deleted. The spliceosome cannot use that missing acceptor.
3. The spliceosome searches for the next available 3' acceptor downstream. The next functional acceptor is at the end of intron 4 (just before exon5).
4. As a result, the spliceosome pairs the 5' donor after exon3 with the 3' acceptor before exon5, removing both intron3 and intron4 as a single unit and omitting exon4 from the mature mRNA.
5. The final spliced mRNA thus contains exon1–exon2–exon3–exon5.

Why the correct answer is correct:
- Loss of a 3' splice site typically causes exon skipping of the downstream exon because the upstream donor pairs with the next downstream acceptor, producing exon3–exon5 junction and thus skipping exon4.

Why the other options are incorrect:
- “Exon 1, 2, 3” would imply exon5 is lost or no further splicing occurs; however downstream splice sites remain and exon5’s acceptor can be used.
- “Exon 1, 2” would require removal or failure to join exon3 as well, which is not implicated by loss of intron3’s 3' site.
- “Exon 1, 2, 4, 5” implies exon3 was skipped; the mutation affects the acceptor before exon4, not before exon3, so exon3 remains present.

Therefore the mature mRNA will contain Exon 1, 2, 3, and 5, matching the provided answer.

A gene encodes a pre-mRNA with 5 exons and 4 introns. A mutation eliminates the 3’ splice site from intron 3. What exons would be found in the mRNA after splicing was completed?单项选择题

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