Conclusion: A group II intron is removed through a self-splicing reaction similar to that of nuclear introns.

114.15 Group II introns undergo self-splicing by a different mechanism from that for group I introns. (a) Secondary structure of a group II intron. (b) Self-splicing of group II introns, which is similar to the splicing of nuclear introns.

Group II introns, present in some mitochondrial genes, also have the ability to self-splice. All group II introns fold into similar secondary structures (IFigure 14.15a). The splicing of group II introns is accomplished by a mechanism that has some similarities to the spliceosomal-medi-ated splicing of nuclear genes; splicing takes place through two transesterification reactions that generate a lariat structure (I Figure 14.15b). Because of these similarities, group II introns and nuclear pre-mRNA introns have been suggested to be evolutionarily related—perhaps the nuclear introns evolved from self-splicing group II introns and later adopted the proteins and snRNAs of the spliceosome to carry out the splicing reaction.

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