Once excision of the intron and splicing of the adjacent exons are completed, the story is over. Link to a discussion of transposons. I introns share a characteristic secondary structure and mode of action that distinguishes them from the next group. Because their secondary structure and the details of the splicing reaction differ from the rRNA introns discussed above, these are called Group II introns. The molecular details of the reactions are similar to those of Group II introns, and this has led to speculation that this splicing machinery evolved from them.
Some viroidlike molecules get into the cell as passengers inside a conventional plant virus.
These are called virusoids or viroidlike satellite RNAs . In both cases, the molecules consists of single-stranded RNA whose ends are covalent bonded to form a circle. There are several regions where base-pairing occurs across adjacent portions of the molecule. New viroids and virusoids are synthesized by the host cell as long precursors in which the viroid structure is tandemly repeated. These repeats must be cut out and ligated to form the final product.