Figure 1. (a) Intron removal involves two trans-esterification reactions. First, the branch point adenosine within the intron cleaves the phosphodiester bond at the 5¢-end of the intron, and a new phosphodiester bond is formed to produce a lariat intermediate. Secondly, the other intermediate product, exon 1, attacks the phosphodiester bond at the 3¢SS to produce the spliced mRNA. The intron is excised as a lariat byproduct that is processed further. (b) Three different types of intron sequences are recognized by different groups of snRNPs. The conserved intronic sequences are shown above each type of intron, Yn represents a polypyrimidine tract and R means a purine. The different 5¢SS and branchpoint sequences determine which spliceosome they will be spliced by.

Figure 2. Simplified schematic showing the conserved snRNP rearrangements during spliceosome assembly and activation.

Figure 3. Schematic of the early stages of spliceosome assembly. The black shapes represent DExH/D proteins that promote rearrangements of RNA–protein interactions at the branchpoint adenosine.

Figure 4. A schematic illustrating the involvement of SFs in intron commitment and/or exon definition on a pre-mRNA. CPC, cap-binding complex; ESE (striped block), exon splicing enhancer.

Figure 5. Schematic showing the cycle of assembly and disassembly of the U2-spliceosomal machinery upon pre-mRNA. The black shapes represent the DExD/H box proteins, their positions indicating their functional requirement at specific spliceosomal stages.