The splicing of pre-mRNA occurs within a large, very dynamic complex, designated the 'spliceosome'. The 50-60S spliceosomes are estimated to be 40-60 nm in diameter, and have molecular weights in the range of 3-5 million kDa. Small nuclear RNAs (snRNAs) U1, U2, U4, U5, and U6, are some of the best characterized components of spliceosomes, and are known to play key roles not only in spliceosomal assembly, but also in the two catalytic steps of the splicing reaction. Over 150 proteins have been detected in spliceosomes, and only a subset of these has been characterized. The characterization, and the determination of the functions of the protein components of the spliceosome, is still work in progress.
During spliceosome assembly, the snRNAs and the spliceosomal proteins assemble on the pre-mRNA in a stepwise pathway. First the E complex forms, followed by complexes A and B; the C complex forms next and contains the products of the first step of the splicing reaction. Complexes called i and D form as a consequence of the second step of the splicing reaction, which contain the excised intron and the spliced exons, respectively.