Question

Which of the following consensus sequences are NOT found in nuclear introns?

1) GU at the 5′ splice site at the beginning of the intron
2) AG at the 3′ splice site at the end of the intron
3) CCA at the 3′ site downstream of the branch point
4) A at the lariat branch point site
5) 3′ CAGG consensus sequence at the 3′ splice junction

Answer 1

The sequence 3' CCA at the 3' site downstream of the branch point is not found in nuclear introns. This sequence does not match with the known consensus sequences necessary for mRNA splicing.

Step-by-step explanation:

The consensus sequences found in nuclear introns are important for the splicing process, wherein non-protein-coding intervening sequences are precisely removed from mRNA before protein synthesis. Among the listed options, the sequence that is not found in the nuclear introns is 3' CCA at the 3' site downstream of the branch point. This sequence does not align with the established consensus sequences found at either the 5' or 3' ends of introns or the branch point within the intron, which are typically GU at the 5' splice site, AG at the 3' splice site, and an A at the branch point site.