Pre-mRNA Processing: RNA Splicing

When pre-mRNA is transcribed from DNA, it contains exons, protein-coding sequences, and introns, non-coding regions. RNA splicing removes the introns and attaches the exons together.

This is catalyzed at the spliceosome, a large assembly of small nuclear ribonucleoproteins, or snRNPs — complexes made up of small nuclear RNAs and proteins.

The spliceosome is composed of 5 snRNPs, U1, U2, U4, U5 and U6, and several other proteins.

First, U1 snRNP binds the 5' splice site while the branch point sequence containing A is recognized by other proteins and later replaced by U2 snRNP. 

Next, a complex of U4/U6 and U5 snRNPs binds to the 5' splice junction and assists in bringing the 5’ end to the branch point A. A transesterification reaction between the branch point A and the 5' splice site creates a loop called a lariat.

Following this, another transesterification reaction occurs between the 3' end of the upstream exon and the 5' end of the next exon. This cleaves off the lariat containing the intron, leaving the exons attached to each other.