text: Splicing is catalyzed by the spliceosome, a large RNA-protein complex composed of five small nuclear ribonucleoproteins (snRNPs).  Assembly and activity of the spliceosome occurs during transcription of the pre-mRNA.  The RNA components of snRNPs interact with the intron and are involved in catalysis.  Two types of spliceosomes have been identified (major and minor) which contain different snRNPs.
  *The major spliceosome splices introns containing GU at the 5 splice site and AG at the 3 splice site.  It is composed of the U1, U2, U4, U5, and U6 snRNPs and is active in the nucleus. In addition, a number of proteins including U2 small nuclear RNA auxiliary factor 1 (U2AF35), U2AF2 (U2AF65) and SF1 are required for the assembly of the spliceosome.  The spliceosome forms different complexes during the splicing process:
  :*Complex E
  :**The U1 snRNP binds to the GU sequence at the 5' splice site of an intron;
  :**Splicing factor 1 binds to the intron branch point sequence;
  :**U2AF1 binds at the 3' splice site of the intron;
  :**U2AF2 binds to the polypyrimidine tract;
  :*Complex A (pre-spliceosome)
  :**The U2 snRNP displaces SF1 and binds to the branch point sequence and ATP is hydrolyzed;
  :*Complex B (pre-catalytic spliceosome)
  :**The U5/U4/U6 snRNP trimer binds, and the U5 snRNP binds exons at the 5' site, with U6 binding to U2;
  :*Complex B*
  :**The U1 snRNP is released, U5 shifts from exon to intron, and the U6 binds at the 5' splice site;
  :*Complex C (catalytic spliceosome)
  :**U4 is released, U6/U2 catalyzes transesterification, making the 5-end of the intron ligate to the A on intron and form a lariat, U5 binds exon at 3 splice site, and the 5' site is cleaved, resulting in the formation of the lariat;
  :*Complex C* (post-spliceosomal complex)
  :**U2/U5/U6 remain bound to the lariat, and the 3' site is cleaved and exons are ligated using ATP hydrolysis.  The spliced RNA is released, the lariat is released and degraded, and the snRNPs are recycled.
  :This type of splicing is termed canonical splicing or termed the lariat pathway, which accounts for more than 99% of splicing. By contrast, when the intronic flanking sequences do not follow the GU-AG rule, noncanonical splicing is said to occur (see "minor spliceosome" below).
  *The minor spliceosome is very similar to the major spliceosome, but instead it splices out rare introns with different splice site sequences. While the minor and major spliceosomes contain the same U5 snRNP, the minor spliceosome has different but functionally analogous snRNPs for U1, U2, U4, and U6, which are respectively called U11, U12, U4atac, and U6atac.