Ag

3'consensus sequence

Conclusion: Critical consensus sequences are present at the 5' splice site, the branch point, and the 3' splice site.

In the first step, the pre-mRNA is cut at the 5' splice site. This cut frees exon 1 from the intron, and the 5' end of the intron attaches to the branch point; that is, the intron folds back on itself, forming a structure called a lariat. The guanine nucleotide in the consensus sequence at the 5' splice site bonds with the adenine nucleotide at the branch point. This bonding is accomplished through transesterifi-cation, a chemical reaction in which the OH group on the 2'-carbon atom of the adenine nucleotide at the branch point attacks the 5' phosphodiester bond of the guanine nucleotide at the 5' splice site, cleaving it and forming a new 5'-2' phosphodiester bond between the guanine and adenine nucleotides.

In the second step of RNA splicing, a cut is made at the 3' splice site and, simultaneously, the 3' end of exon

1 becomes covalently attached (spliced) to the 5' end of exon 2. This bond also forms through a transesterification reaction, in which the 3'-OH group attached to the end of exon 1 attacks the phosphodiester bond at the 3' splice site, cleaving it and forming a new phosphodiester bond between the 3' end of exon 1 and the 5' end of exon 2; the intron is released as a lariat. The intron becomes linear when the bond breaks at the branch point and is then rapidly degraded by nuclear enzymes. The mature mRNA consisting of the exons spliced together is exported to the cytoplasm where it is translated.

Although splicing is illustrated in Figure 14.11 as a two-step process, the reactions are in fact coordinated within the spliceosome. A key feature of the spliceosome is a series of interactions between the mRNA and snRNAs and

5 ' splice site

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