spliceosome

Almost all eukaryotic protein-coding genomes contain non-coding intervening sequences called introns. Spliceosomes are complex ribonuclear machines in eukaryotes that remove the non-coding introns from primary transcript, precursor mRNA (pre-mRNA or hnRNA). Alternatively, the term spliceome can be used to describe the complete set of all possible alternative splices in an organism, analogous to the genome or proteome.

Spliceosomes are variably composed of as many as 300 distinct proteins and five RNAs, making them among the most complex macromolecular machines known. review article abstract. Essential components of the spliceosome are the small RNA-protein complexes called small nuclear ribonucleoproteins (snRNPs, pronounced 'snurps'). These are U1, U2, U4, U5, and U6, so named because they are rich in uridine nucleotides. In addition to snRNPs, splicing requires many non-snRNP protein factors. The snRNPs participate in several RNA-RNA and RNA-protein interactions.

The spliceosome recognizes specific 5' and 3' sites on the pre-mRNA. The intronal area between these locations is excised, and the two exons are spliced (ligated). diagram - formation of a spliceosome : diagram - intron excision in mRNA precursors : diagram - pre-mRNA processing : animation ~ alternative splicing : animation of RNA splicing requires Flash Player plugin - Download plugin: clickable slide show - spliceosome intron removal : alternative splicing - click on fig 1 for animation : life cycle of an mRNA ~ click on Quicktime Q : clickable slide show - spliceosome intron removal :

Aberrant splicing creates mutant proteins, while alternative splicing generates diversity. Splice variants and epigenetic mechanisms account for the ability of about 25,000 human genes to code for about 100,000 human proteins. In addition to this variation, and that provided by recombination, each human gene possesses at least two isoforms – one from each parent. X Inactivation