What is a spliceosome and what role does it play in gene expression?

• A. The spliceosome is a ribosome involved in translation.
• B. The spliceosome is a complex of snRNPs and proteins that removes introns and ligates exons during pre-mRNA splicing.
• C. The spliceosome is a DNA polymerase that synthesizes DNA.
• D. The spliceosome is a chaperone for protein folding.

Answer: (B)

Splicing of pre-mRNA is how cells turn a transcript containing both coding exons and noncoding introns into a mature mRNA that can be read correctly by the protein-making machinery. The spliceosome is the complex of small nuclear ribonucleoproteins (snRNPs) and proteins that carries out this task. It recognizes the splice sites at intron borders and drives two catalytic steps: first, cutting at the 5' splice site and forming a lariat structure with the intron; second, cutting at the 3' splice site and ligating the exons together. The RNA components of the snRNPs guide the recognition of splice sites through base pairing, while proteins help rearrange the complex and catalyze the reactions. This processing is essential for gene expression because only mature mRNA, with introns removed and exons joined in the correct order, can serve as the template for translation. Missteps in splicing can prevent proper protein production or create alternative isoforms. The other options describe different cellular machines: a ribosome builds proteins from mRNA, a DNA polymerase copies DNA, and chaperones assist protein folding.