Question

During pre-mRNA splicing reaction, a lariat RNA is formed when the intron cleaved at the 5' splice site gets linked by a

• A. 5' 2' bond to a base within the intron
• B. 5' 3' bond to a base within the intron
• C. 5' 2' bond to a base at the 5' end of the immediate downstream exon
• D. 5' 3' bond to a base at the 5' end of the immediate downstream exon

Hint:

During RNA splicing, the lariat structure forms when the 5' end of the intron forms a 5' 2' phosphodiester bond with a branch point adenosine in the intron.

Answer 1

The Correct Option is A

Step 1: Understanding the process of lariat formation in RNA splicing.
During the splicing of pre-mRNA, the intron is excised and forms a lariat structure. The 5' splice site of the intron reacts with a branch point adenosine (located within the intron) at the 2' hydroxyl group, forming a 5' 2' phosphodiester bond. This lariat structure is a key feature of RNA splicing.

Step 2: Analyzing the options.
(A)5' 2' bond to a base within the intron: Correct — The lariat structure is formed when the 5' end of the intron is covalently linked to a branch point adenosine via a 5' 2' bond, resulting in the excision of the intron.
(B)5' 3' bond to a base within the intron: This is incorrect, as it does not describe the typical 5' 2' linkage formed during lariat formation.
(C)5' 2' bond to a base at the 5' end of the immediate downstream exon: Incorrect — The lariat formation involves bonding within the intron, not with the downstream exon.
(D)5' 3' bond to a base at the 5' end of the immediate downstream exon: This is incorrect because the lariat formation does not involve such a bond.

Step 3: Conclusion.
The correct answer is (A)5' 2' bond to a base within the intron, as this describes the formation of the lariat structure during pre-mRNA splicing.