24. During m-RNA processing splicing is mediated by-
(1) sn RNA           (2) sno-RNA
(3) r-RNA             (4) t-RNA

The correct answer is (1) snRNA.

Explanation:

During pre-mRNA processing, splicing is mediated by the spliceosome, a large ribonucleoprotein complex composed primarily of small nuclear RNAs (snRNAs) and associated proteins. These snRNAs—specifically U1, U2, U4, U5, and U6—are essential for recognizing splice sites, catalyzing the splicing reactions, and ensuring accurate removal of introns from pre-mRNA.

• snRNAs form small nuclear ribonucleoproteins (snRNPs) that assemble on the pre-mRNA to carry out splicing.

• U1 snRNA recognizes the 5′ splice site by base pairing.

• U2 snRNA base pairs with the branch point sequence.

• U4, U5, and U6 snRNAs participate in spliceosome assembly and catalysis.

• The catalytic core of the spliceosome involves U2 and U6 snRNAs.

In contrast:

• snoRNAs (small nucleolar RNAs) are involved in modification and processing of rRNAs, not splicing.

• rRNAs (ribosomal RNAs) are structural and catalytic components of ribosomes, unrelated to splicing.

• tRNAs (transfer RNAs) are involved in translation, not RNA splicing.

Summary

Conclusion

Splicing of pre-mRNA is mediated by snRNAs, which are integral parts of the spliceosome complex responsible for intron removal and exon ligation during mRNA processing