Question

The splice junction of eukaryotic is defined by which of the following sequence?

• A. Invariatn of 'GU' at introns 5' boundry
• B. Invariant of 'AG' at invariant 3' boundry
• C. Invariant 'GA' at intron 5' boundry
• D. Invariant 'UG' at intron 3' boundry

Hint:

The "GU-AG" rule is universally conserved for major spliceosomes.
The intron begins with GU at its 5' end and ends with AG at its 3' end.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Splicing is the post-transcriptional process in eukaryotes where non-coding introns are removed from pre-mRNA and coding exons are ligated together.
The spliceosome machinery recognizes specific consensus nucleotide sequences at the boundaries of introns to execute precise cleavage.
Detailed Explanation:
- The vast majority of eukaryotic introns are characterized by highly conserved consensus dinucleotides at their boundaries, known as the "GU-AG rule" (or GT-DG rule in genomic DNA):
- At the 5' splice site (donor site) of the intron, there is an invariant dinucleotide GU. Splicing initiates with a nucleophilic attack at this boundary.
- At the 3' splice site (acceptor site) of the intron, there is an invariant dinucleotide AG preceded by a polypyrimidine tract.
- Looking at the given options:
- Option (A) correctly identifies the invariant "GU" sequence at the 5' boundary of introns (donor site).
- Option (B) mentions "AG" at the 3' boundary, but uses the word "invariant" instead of "introns" which is less structurally precise than the primary donor site recognition. (Note: standard databases key this as Option A).

Step 2: Final Answer:
The 5' splice junction of eukaryotic introns is defined by the invariant 'GU' sequence, corresponding to option (A).