18. Pre-mRNAs are rapidly bound by snRNPs which carry out dual steps of splicing that removes the intron and joins the upstream and downstream exons. The following statements describe some facts related to this event
A. Almost all introns begin with GU and end with AG sequences and hence all the GU or AG sequences are spliced out of RNA.
B. U2 RNA recognizes important sequences at the 3′ acceptor end of the intron.
C. The spliceosome uses ATP to carry out accurate removal of intron
D. An unusual linkage with 2′ OH group of adenosine within intron form ‘Lariat’ structure.
Which of the following combinations is correct?
(1) A and B     (2) B and C
(3) C and D     (4) D and A

Understanding Pre-mRNA Splicing: Key Facts About snRNPs and the Spliceosome

Among the given statements about pre-mRNA splicing, the correct combination is:

(3) C and D

Explanation of Each Statement

A. Almost all introns begin with GU and end with AG sequences and hence all the GU or AG sequences are spliced out of RNA.

• This is incorrect. While most introns do begin with a GU at the 5′ splice site and end with an AG at the 3′ splice site (the canonical splice sites), not all GU or AG sequences in RNA are spliced out. Only those at the precise splice junctions are recognized and removed. GU and AG sequences elsewhere in the RNA are not necessarily spliced.

B. U2 RNA recognizes important sequences at the 3′ acceptor end of the intron.

• This is partially incorrect. U2 snRNA binds to the branch point sequence near the 3′ end of the intron, but it does not directly recognize the 3′ splice site (acceptor site) itself. The 3′ splice site is recognized by other factors like U2AF.

C. The spliceosome uses ATP to carry out accurate removal of intron.

• This is correct. Splicing is an ATP-dependent process. ATP hydrolysis is required for spliceosome assembly, rearrangements, and catalysis to ensure accurate intron removal.

D. An unusual linkage with 2′ OH group of adenosine within intron forms ‘Lariat’ structure.

• This is correct. During the first step of splicing, the 2′ hydroxyl group of a conserved adenine at the branch point attacks the 5′ splice site, forming a 2′–5′ phosphodiester bond and creating a lariat-shaped intermediate.

Summary

• Splicing involves precise recognition of splice sites, not all GU or AG sequences.

• U2 snRNA binds the branch point, not directly the 3′ splice site.

• ATP is essential for spliceosome function.

• The lariat structure is formed via a 2′ OH linkage at the branch point adenine.

Therefore, the correct combination of true statements is:

(3) C and D