What is a spliceosome?

• A. A ribonucleoprotein complex that catalyzes intron removal and exon ligation.
• B. A protein complex that inspects mRNA quality.
• C. A ribonucleoprotein complex that catalyzes intron removal and exon ligation.
• D. A ribonucleoprotein complex that catalyzes ribosome assembly.

Answer: (C)

Spliceosome is the cellular machine that carries out splicing of pre-mRNA in the nucleus. It is a ribonucleoprotein complex, meaning it contains RNA and protein components, with small nuclear RNAs forming the catalytic core. It recognizes splice sites, removes introns, and joins exons through two transesterification reactions, producing mature mRNA. The best description is that it catalyzes intron removal and exon ligation. The other options point to processes that are not splicing: one describes mRNA quality control, and the other ribosome assembly.