Which molecular machine excises introns and splices exons in most eukaryotic pre-mRNA?

• A. The spliceosome.
• B. The ribosome.
• C. The proteasome.
• D. The polymerase.

Answer: (A)

The process being tested is mRNA processing in eukaryotes, where introns are removed and exons joined to create a mature message. This work is done by the spliceosome, a large complex made of RNA and proteins (including snRNPs like U1, U2, U4, U5, and U6). It recognizes the splice sites at intron–exon boundaries and carries out two coordinated reactions: first, the 5′ end of the intron is cut and attached to a branch-point adenine to form a lariat; second, the intron is released and the exons are ligated together. The result is a continuous coding sequence, often with alternative splicing generating different protein variants. The ribosome translates mature mRNA, the proteasome degrades proteins, and polymerase is involved in transcription, not splicing.