Which RNA processing step removes introns from pre-mRNA in eukaryotic cells?

• A. Splicing.
• B. 5' capping.
• C. 3' polyadenylation.
• D. Translation.

Answer: (A)

Splicing is the RNA processing step that removes introns from pre-mRNA in eukaryotic cells. After transcription, the pre-mRNA is modified in the nucleus by adding a 5' cap and a 3' poly(A) tail, while introns are removed through the action of the spliceosome. The spliceosome recognizes intron-exon boundaries, cuts out the introns as a looped structure, and ligates the exons together to form mature mRNA. This mature mRNA is then exported to the cytoplasm for translation. Splicing also enables diversity through alternative splicing, where different exons can be included or skipped to produce multiple protein variants from a single gene. The other processes listed are distinct steps: 5' capping adds the protective cap at the start of the transcript; 3' polyadenylation adds the poly(A) tail; translation is the synthesis of protein from mRNA.