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mRNA splicing - major pathway in Homo sapiens

mRNA splicing - major pathway in KEGG: hsa03040
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mRNA splicing - major pathway in REACTOME: REACT_467

The process in which excision of introns from the primary transcript of messenger RNA (mRNA) is followed by ligation of the two exon termini exposed by removal of each intron, is called mRNA splicing. Most of the mRNA is spliced by the major pathway, involving the U1, U2, U4, U5 and U6 snRNPs. A minor fraction, about 1 %, of the mRNAs are spliced via the U12 dependent pathway. [source: REACTOME - REACT_1735.2]

Major pathway: 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. [source: REACTOME - REACT_467.2]

Information about proteins and complexes taking part in splicing can be found in SpliProt3D or in Spliceosome Database.

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Last modification of this entry: Sept. 18, 2012.

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