circles) plus other proteins (not shown). One function of the complex of snRNPs is to bring the two ends of the intron together so that the reaction can take place. After the assembly of the snRNPs, a specific adenine nucleotide in the intron sequence (indicated in red) attacks the 5?splice site and cuts the sugar-phosphate backbone of the RNA at this point. The cut 5?end of the intron becomes covalently linked to the adenine nucleotide, forming a loop, or lariat, in the RNA molecule. The free 3?OH end of the first exon sequence then reacts with the beginning of the second exon sequence, cutting the intron at its 3?end and, at the same time, joining the two exons together. The outcome of these splicing reactions is that the two exon sequences become joined into a continuous coding sequence, and the lariat containing the intron sequence is released and eventually degraded
