RNA Splicing Mechanism

RNA splicing is a process in molecular biology where a newly-made precursor messenger RNA (pre-mRNA) transcript is transformed into a mature messenger RNA (mRNA). It works by removing all the introns (non-coding regions of RNA) and splicing back together exons (coding regions). For nuclear-encoded genes, splicing occurs in the nucleus either during or immediately after transcription. For those eukaryotic genes that contain introns, splicing is usually needed to create an mRNA molecule that can be translated into protein. For many eukaryotic introns, splicing occurs in a series of reactions which are catalyzed by the spliceosome, a complex of small nuclear ribonucleoproteins (snRNPs). There exist self-splicing introns, that is, ribozymes that can catalyze their own excision from their parent RNA molecule. The process of transcription, splicing and translation is called gene expression, the central dogma of molecular biology.
Splicing is catalyzed by the spliceosome, a large RNA-protein complex composed of five small nuclear ribonucleoproteins (snRNPs). Assembly and activity of the spliceosome occurs during transcription of the pre-mRNA. The RNA components of snRNPs interact with the intron and are involved in catalysis. Two types of spliceosomes have been identified (major and minor) which contain different snRNPs.

The major spliceosome splices introns containing GU at the 5' splice site and AG at the 3' splice site. It is composed of the U1, U2, U4, U5, and U6 snRNPs and is active in the nucleus. In addition, a number of proteins including U2 small nuclear RNA auxiliary factor 1 (U2AF35), U2AF2 (U2AF65)[10] and SF1 are required for the assembly of the spliceosome.The spliceosome forms different complexes during the splicing process

Complex E
The U1 snRNP binds to the GU sequence at the 5' splice site of an intron;
Splicing factor 1 binds to the intron branch point sequence;
U2AF1 binds at the 3' splice site of the intron;
U2AF2 binds to the polypyrimidine tract;[13]
Complex A (pre-spliceosome)
The U2 snRNP displaces SF1 and binds to the branch point sequence and ATP is hydrolyzed;
Complex B (pre-catalytic spliceosome)
The U5/U4/U6 snRNP trimer binds, and the U5 snRNP binds exons at the 5' site, with U6 binding to U2;
Complex B*
The U1 snRNP is released, U5 shifts from exon to intron, and the U6 binds at the 5' splice site.
Complex C (catalytic spliceosome)
U4 is released, U6/U2 catalyzes transesterification, making the 5'-end of the intron ligate to the A on intron and form a lariat, U5 binds exon at 3' splice site, and the 5' site is cleaved, resulting in the formation of the lariat;
Complex C* (post-spliceosomal complex)
U2/U5/U6 remain bound to the lariat, and the 3' site is cleaved and exons are ligated using ATP hydrolysis. The spliced RNA is released, the lariat is released and degraded,[14] and the snRNPs are recycled.