Editing Intron (section)

=== Group I and group II introns ===
{{see also|Group I catalytic intron|Group II intron}}
Group I and group II introns are found in genes encoding proteins ([[messenger RNA]]), [[transfer RNA]] and [[ribosomal RNA]] in a very wide range of living organisms.<ref>{{cite journal | vauthors = Cech TR | title = Self-splicing of group I introns | journal = Annual Review of Biochemistry | volume = 59 | pages = 543–568 | year = 1990 | pmid = 2197983 | doi = 10.1146/annurev.bi.59.070190.002551 }}</ref><ref>{{cite journal | vauthors = Michel F, Ferat JL | title = Structure and activities of group II introns | journal = Annual Review of Biochemistry | volume = 64 | pages = 435–461 | year = 1995 | pmid = 7574489 | doi = 10.1146/annurev.bi.64.070195.002251 }}</ref> Following transcription into RNA, group I and group II introns also make extensive internal interactions that allow them to fold into a specific, complex [[Nucleic acid tertiary structure|three-dimensional architecture]]. These complex architectures allow some group I and group II introns to be ''self-splicing'', that is, the intron-containing RNA molecule can rearrange its own covalent structure so as to precisely remove the intron and link the exons together in the correct order. In some cases, particular intron-binding proteins are involved in splicing, acting in such a way that they assist the intron in folding into the three-dimensional structure that is necessary for self-splicing activity. Group I and group II introns are distinguished by different sets of internal conserved sequences and folded structures, and by the fact that splicing of RNA molecules containing group II introns generates branched introns (like those of spliceosomal RNAs), while group I introns use a non-encoded guanosine nucleotide (typically GTP) to initiate splicing, adding it on to the 5'-end of the excised intron.