Following statements were made about mRNA splicing:

(A) Involvement of a cis-acting branchpoint site (or branchpoint sequence) present near the 3' end of each exon is essential for splicing.

(B) In the first step of the splicing reaction, 2'-OH of the conserved U at the branch-site acts as a nucleophile to attack the phosphoryl group of the conserved G in the 5' splice site.

(C) The newly liberated intron adapts the shape of a lariat due to the joining of the 5' end of the intron to the branchpoint.

(D) During the splicing process, there is no net gain in the number of chemical bonds.

(E) Prp22 (a DEAD-box helicase) is required for stripping the spliced mRNA from the spliceosome.

Which one of the following options shows the correct combination of all true statements?

​

Correct option is B

​Explanation:

• Statement A (Incorrect): The branchpoint sequence is located within the intron, not at the 3' end of an exon. While the branchpoint is crucial for splicing, the positioning in the statement is incorrect.
• Statement B (Incorrect): The first step of splicing involves a nucleophilic attack by the 2'-OH of an adenine (A) at the branch-site, not a conserved uracil (U). The mention of U instead of A makes it incorrect.
• Statement C (Correct): The lariat structure is formed when the 5' end of the intron covalently joins to the branchpoint A, making it a correct statement.
• Statement D (Correct): Splicing does not create or destroy net chemical bonds but instead rearranges phosphodiester linkages, meaning no net gain occurs.
• Statement E (Correct): Prp22, a DEAD-box helicase, plays a crucial role in removing the spliced mRNA from the spliceosome.

Thus, the correct combination of true statements is C, D, and E, making Option (2) the correct answer.

Information Booster:

1. Pre-mRNA splicing removes introns and joins exons to form mature mRNA.
2. The branchpoint sequence is a conserved intron sequence, NOT at the exon-intron junction.
3. Lariat formation results from the 2'-5' linkage between the branchpoint A and the 5' end of the intron.
4. Spliceosome complex includes snRNPs (U1, U2, U4, U5, U6), which regulate splicing.
5. Prp22 helicase is essential for spliced mRNA release from the spliceosome.
6. Defective splicing can lead to Spinal Muscular Atrophy (SMA) and β-thalassemia.

Additional Knowledge:

(A) Branchpoint Sequence and Splicing

• The branchpoint site is present within the intron, NOT at the 3' end of an exon.
• It contains a conserved adenine (A), essential for initiating the first splicing reaction.
• Mutations in this sequence can cause abnormal splicing and disease.

(B) First Step of Splicing and Nucleophilic Attack

• The first step of pre-mRNA splicing involves a nucleophilic attack by the 2'-OH of an adenine (A) at the branch-site.
• This attack breaks the 5' splice site, forming a lariat intermediate.
• Since the statement incorrectly mentions U instead of A, it is false.

(C) Lariat Formation in Splicing

• The lariat structure forms due to the covalent bond between the 5' end of the intron and the branchpoint A.
• This is a temporary structure before intron degradation.

(D) Chemical Bond Rearrangement in Splicing

• Splicing does not add or remove net chemical bonds.
• It only rearranges phosphodiester linkages, ensuring correct exon joining.

(E) Role of Prp22 in mRNA Splicing

• Prp22 is a DEAD-box helicase that removes spliced mRNA from the spliceosome.
• It uses ATP hydrolysis to release the mature transcript.
• Mutations in Prp22 can trap mRNA within the spliceosome, disrupting translation.

​