The process by which an intron is excised and exons are religated in the post-transcriptional modification of RNA. The process is effected in vivo by a complex, called a spliceosome, of small nuclear RNA-proteins (snrps) and other proteins that assemble on a pre-mRNA and by hybridizing with the pre-mRNA at splice site consensus sequences, force the appropriate regions of the pre-mRNA into a reactive configuration and catalyze the excision of an intron in a process mechanistically similar to group II self-splicing. Snrps act by a two-step mechanism that cleaves the phosphodiester bond between the intron and the upstream exon, leaving a 3'-hydroxyl group; the 5'-phosphate of the intron is transferred to the 2'-hydroxyl usually of an adenine nucleotide (the branch point) of the intron near its 5' limit; the 3'-hydroxyl of the upstream exon then displaces the 3'-terminal nucleotide of the intron from the downstream exon, thus ligating the two exons and leaving the intron as a lariat, a rho-shaped structure that with the 5'- terminus anchored to the branch point. In vivo, the process is catalyzed by large nuclear ribonucleoprotein particles (lnRNPs), 200 S particles (500 Å, approx. 500 KDa), composed of four subunits, each of which appears to perform like a snrp.