RNA splicing: processing of pre-mRNA into mature messenger RNA

Basic mechanism

Pre-mRNA produced by a gene contains alternating coding and non-coding regions. The coding regions are called exons and the intervening non-coding regions are introns. During splicing, introns are removed and exons are joined. The reaction proceeds through two transesterification steps carried out by a large RNA–protein complex known as the spliceosome. The spliceosome recognizes short conserved sequences at intron boundaries and catalyzes intron excision and exon ligation to yield a contiguous open reading frame.

Key components and signals

• Introns and exons: introns are non-coding stretches that must be removed; exons remain in the mature message. (intron)
• Spliceosome: formed from small nuclear RNAs and many proteins; it assembles dynamically on the pre-mRNA to execute splicing. (spliceosome)
• Cis-acting signals: short sequence motifs at the 5' splice site, branch point, polypyrimidine tract and 3' splice site guide accurate cutting and joining.
• Trans-acting factors: regulatory proteins and snRNPs that influence splice-site selection and efficiency.

Steps in the splicing cycle

1. Recognition: splice sites and branch point are identified by snRNPs and protein factors.
2. Assembly: spliceosomal subunits assemble into an active complex on the pre-mRNA.
3. Catalysis: two sequential transesterification reactions remove the intron as a lariat and join exons.
4. Disassembly and recycling: components dissociate and are reused for new splicing events.

Splicing is discussed widely within molecular biology because it affects the fidelity and regulation of gene expression. Errors in splice-site recognition or catalysis can alter reading frames or produce truncated proteins, with potential consequences for cell function and health.

Biological significance and diversity

Beyond producing a single mature transcript, splicing contributes to transcriptome complexity through alternative splicing. Alternative patterns allow a single gene to generate multiple mRNA variants with different exon combinations, expanding the repertoire of possible proteins. This regulation is tissue-specific and responsive to developmental cues and environmental signals, making splicing a major source of proteomic diversity and regulatory control.

Historical and clinical notes

The recognition that many eukaryotic genes are discontinuous and require removal of internal segments was a turning point in understanding gene architecture. Since that discovery, splicing has become an important area in medical research: many inherited disorders and cancers are linked to splicing defects, and therapeutic approaches now target splicing to correct aberrant messages or alter isoform production.

For further reading about the relation of transcription and splicing, or the molecular machines that carry out these steps, see introductory resources on transcription, gene structure (gene), introns (intron), spliceosome function (spliceosome), and the downstream processes of translation (translation) and protein synthesis (protein). Additional background in molecular biology textbooks can place splicing in the broader context of cellular information flow.