Which molecular complex is primarily involved in splicing introns from pre-mRNA in eukaryotes?

• A. Spliceosome
• B. Ribosome
• C. Polymerase II complex
• D. Telomerase

Answer: (A)

Spliceosome-mediated RNA processing is the key process here. In eukaryotes, pre-mRNA contains introns that must be removed to form mature mRNA. The spliceosome, a large complex of small nuclear RNAs and proteins, recognizes the 5' splice site, the branch point, and the 3' splice site. It catalyzes two transesterification reactions: first, the 5' end of the intron is cut and attached to the branch point to form a lariat; second, the exons are joined and the intron is released as a lariat. This precise removal and exon ligation create a continuous coding sequence for translation. The other complexes have different roles: the ribosome translates mature mRNA, RNA polymerase II synthesizes RNA from DNA, and telomerase extends telomeres.