17. Which of following is reactive centre for splicing of exons during processing of m-RNA?
(1) U1 and U5
(2) Branch point, U2 and U6
(3) Branch point, U4 and U6
(4) U2 and U4

The reactive center for splicing of exons during eukaryotic mRNA processing involves the branch point sequence and the small nuclear RNAs U2 and U6.

Explanation:

• The branch point is a conserved adenine nucleotide within the intron that plays a crucial role in the first step of splicing by forming a lariat structure.

• U2 snRNA base pairs with the branch point sequence, helping to position it for catalysis.

• U6 snRNA interacts with U2 and the 5′ splice site to form the catalytic core of the spliceosome.

This U2-U6 complex forms the active site where the two transesterification reactions of splicing occur, making them essential components of the spliceosome’s catalytic center.

Other snRNAs like U1 and U5 have important roles—U1 recognizes the 5′ splice site initially, and U5 aligns the exons for ligation—but the catalytic reactive center is primarily formed by the branch point, U2, and U6.

Correct answer:

(2) Branch point, U2 and U6

Supporting points from references:

• U2 snRNA pairs with the branch point sequence and is critical for branch site recognition.

• U6 snRNA forms base-pairing interactions with U2 and the 5′ splice site, contributing to catalysis.

• The branch point adenine attacks the 5′ splice site to form the lariat intermediate, a key step in splicing.

This combination represents the reactive center essential for the splicing reaction during eukaryotic mRNA processing.