PremRNA

Pre-mRNA, or precursor messenger RNA, is the initial RNA transcript produced by RNA polymerase II from a gene in eukaryotes. It contains both exons, which encode protein sequences, and introns, which must be removed. Before export to the cytoplasm, pre-mRNA is processed by 5' capping, 3' end formation with polyadenylation, and intron removal by splicing. The 5' cap is a modified guanine nucleotide added early in transcription; the poly(A) tail is added after cleavage of the 3' end. These modifications stabilize the transcript, promote nuclear export, and enhance translation. Processing is often co-transcriptional, involving the spliceosome, snRNPs, and numerous RNA-binding proteins.

Splicing excises introns and ligates exons to produce a mature mRNA. Splicing uses conserved junction signals and a lariat mechanism; the exon junction complex marks spliced mRNAs for export and surveillance. Alternative splicing can generate multiple mRNA and protein isoforms from a single gene. 3' end formation requires cleavage and polyadenylation factors such as CPSF and CstF; the poly(A) tail influences stability and translational efficiency. The mature mRNA is exported to the cytoplasm by export receptors, and translation proceeds. Quality control mechanisms, including nonsense-mediated decay, monitor transcripts for premature stop codons and degrade faulty mRNAs.