Answer:
The correct answer is a. Exon/intron boundaries are typically characterized by a 5' GU splice junction and a 3' AG splice junction.
Explanation:
Splicing is the process in which the non-coding introns are removed from the mRNA and only coding sequences remains in mRNA.
The splicing of introns depends on the recognition of splice site pair. Usually, 5'-GU-3' is 5' splice site and 5'-AG-3' is 3' splice site which makes the boundary of introns/exons and these two nucleotides are important to recognize the introns.
There are some additional conserved sequence at these two splice site which help in recognition of introns. First, a lariate structure is formed when 5'-GU-3' is cleaved and joined to the branch point that is present near 3' slice site. Then 3' splice site is cleaved which releases the intron. Therefore the correct answer is a.
