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RNA splicing is catalyzed by a spliceosome formed from the assembly of U1, U2, U5, and U4/U6 snRNPs (shown as green circles) plus other components (not shown). After assembly of the spliceosome, the reaction occurs in two steps: in step 1 the branch-point A nucleotide in the intron sequence, which is located close to the 3' splice site, attacks the 5' splice site and cleaves it; the cut 5' end of the intron sequence thereby becomes covalently linked to this A nucleotide, forming the branched nucleotide shown in Figure 8-55. In step 2 the 3'-OH end of the first exon sequence, which was created in the first step, adds to the beginning of the second exon sequence, cleaving the RNA molecule at the 3' splice site; the two exon sequences are thereby joined to each other and the intron sequence is released as a lariat. The spliceosome complex sediments at 60S, indicating that it is nearly as large as a ribosome. These splicing reactions occur in the nucleus and generate mRNA molecules from primary RNA transcripts (mRNA precursor molecules).