Intron splicing of nuclear genes is a two-step process: (1) the 5' end of the intron is cleaved and attached to the branch point to form a lariat and (2) the 3' end of the intron is cleaved and the two ends of the exon are spliced together. These reactions take place within the spliceosome.

Self-splicing introns Some introns are self-splicing, meaning that they possess the ability to remove themselves from an RNA molecule. These self-splicing introns fall into two major categories. Group I introns are found in a variety of genes, including some rRNA genes in pro-tists, some mitochondrial genes in fungi, and even some bacteriophage genes. Although the lengths of group I in-trons vary, all of them fold into a common secondary structure with nine looped stems (IFigure 14.14a), which are necessary for splicing. Transesterification reactions are required for the splicing of group I introns (I Figure 14.14b).

All but the exons is removed by splicing.

3' splice site

5' spMce site

DExon 1

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