A spliceosome is an enzyme involved in the process of transcription which requires ATP to catalyse the removal of introns. Because DNA contains a large amount of non-coding information (introns). when mRNA is transcribed it also contains non-coding information. This is of no use to the mRNA as the non-coding information does not need to be transcribed. The spliceosome is the enzyme that splices the introns out of pre-mRNA to give the final mRNA product ready for translation.
The spliceosome contains five snRNAs (small nuclear RNAs), named U1, U2, U3, U4 U5 and U6 that are less than 200 nucleotides each, which perform the splicing procedure. Each snRNA is complexed with several protein subunits, no less than 7, to form a snRNP (small nuclear ribonucleoprotein), and these snRNP molecules form the core of the spliceosome.
During splicing, the spliceosome recognises the target sequences and assembles on the substrate through extensive rearrangement of its components in the pre-mRNA. The re-arrangements have to be very highly co-ordinated and are said to be driven by DExD/H-box ATPases.
According to the classical model of spliceosome assembly (Will and Luhrmann, 2006), when U1 snRNA binds to the splice site, this commits the pre-mRNA to splicing and the U2 snRNA binds to the intronic branch-site to form the pre-spliceosome. The U4/U6 and U5 triple-snRNA is also added to the pre-spliceosome, forming a catalytically inert complex. The next few steps result in mRNA release and spliceosome disassembly.