What is the role of the spliceosome in RNA processing?

• A. The spliceosome catalyzes intron removal and exon ligation to produce mature mRNA.
• B. The spliceosome adds a 5' cap to mRNA.
• C. The spliceosome polyadenylates mRNA.
• D. The spliceosome translates mRNA.

Answer: (A)

Spliceosome’s job is to remove introns and join exons to produce mature mRNA. This large RNA–protein machine recognizes the splice sites and branch point within the pre-mRNA, then catalyzes two sequential chemical steps that cut out the intron and seal the exons together. In the first step, the 2′-hydroxyl of a branch-point nucleotide attacks the 5′ splice site, creating a looped intron (a lariat) and freeing the 5′ exon. In the second step, the free 3′-hydroxyl of the 5′ exon attacks the 3′ splice site, joining the two exons and releasing the intron as the lariat. The result is mature mRNA ready for export and translation.

Other RNA processing events—like adding a 5′ cap to the transcript and adding a poly(A) tail at the 3′ end—are carried out by different enzymes, not the spliceosome, and translation is performed by ribosomes in the cytoplasm. Splicing also underlies alternative splicing, where different combinations of exons can be joined to produce multiple mRNA variants from a single gene.