Editing RNA (section)

===Chemical modifications===
[[File:Ciliate telomerase RNA.JPG|class=skin-invert-image|thumb|upright=1.25|[[Secondary structure]] of a [[telomerase RNA]]]]
RNA is transcribed with only four bases (adenine, cytosine, guanine and uracil),<ref>{{cite book|title=Clinical gene analysis and manipulation: Tools, techniques and troubleshooting|vauthors=Jankowski JA, Polak JM|date=1996|page=[https://archive.org/details/clinicalgeneanal0000unse/page/14 14]|publisher=Cambridge University Press|isbn=978-0-521-47896-0|oclc=33838261|url=https://archive.org/details/clinicalgeneanal0000unse/page/14}}</ref> but these bases and attached sugars can be modified in numerous ways as the RNAs mature. [[Pseudouridine]] (Ψ), in which the linkage between uracil and ribose is changed from a C–N bond to a C–C bond, and [[5-methyluridine|ribothymidine]] (T) are found in various places (the most notable ones being in the TΨC loop of [[tRNA]]).<ref>{{cite journal | vauthors = Yu Q, Morrow CD | title = Identification of critical elements in the tRNA acceptor stem and T(Psi)C loop necessary for human immunodeficiency virus type 1 infectivity | journal = Journal of Virology | volume = 75 | issue = 10 | pages = 4902–6 | date = May 2001 | pmid = 11312362 | pmc = 114245 | doi = 10.1128/JVI.75.10.4902-4906.2001 }}</ref> Another notable modified base is [[hypoxanthine]], a deaminated adenine base whose [[nucleoside]] is called [[inosine]] (I). Inosine plays a key role in the [[wobble hypothesis]] of the [[genetic code]].<ref>{{cite journal | vauthors = Elliott MS, Trewyn RW | title = Inosine biosynthesis in transfer RNA by an enzymatic insertion of hypoxanthine | journal = The Journal of Biological Chemistry | volume = 259 | issue = 4 | pages = 2407–10 | date = February 1984 | doi = 10.1016/S0021-9258(17)43367-9 | pmid = 6365911 | doi-access = free }}</ref>

There are more than 100 other naturally occurring modified nucleosides.<ref>{{cite journal | vauthors = Cantara WA, Crain PF, Rozenski J, McCloskey JA, Harris KA, Zhang X, Vendeix FA, Fabris D, Agris PF | title = The RNA Modification Database, RNAMDB: 2011 update | journal = Nucleic Acids Research | volume = 39 | issue = Database issue | pages = D195-201 | date = January 2011 | pmid = 21071406 | pmc = 3013656 | doi = 10.1093/nar/gkq1028 }}</ref> The greatest structural diversity of modifications can be found in [[tRNA]],<ref>{{cite book|title=TRNA: Structure, biosynthesis, and function| vauthors = Söll D, RajBhandary U |date=1995|page=165|publisher=ASM Press|isbn=978-1-55581-073-3|oclc=183036381 }}</ref> while pseudouridine and nucleosides with [[2'-O-methylation|2'-O-methylribose]] often present in rRNA are the most common.<ref>{{cite journal | vauthors = Kiss T | title = Small nucleolar RNA-guided post-transcriptional modification of cellular RNAs | journal = The EMBO Journal | volume = 20 | issue = 14 | pages = 3617–22 | date = July 2001 | pmid = 11447102 | pmc = 125535 | doi = 10.1093/emboj/20.14.3617 }}</ref> The specific roles of many of these modifications in RNA are not fully understood. However, it is notable that, in ribosomal RNA, many of the post-transcriptional modifications occur in highly functional regions, such as the peptidyl transferase center<ref>{{cite journal | vauthors = Tirumalai MR, Rivas M, Tran Q, Fox GE | title = The Peptidyl Transferase Center: a Window to the Past. | journal = Microbiol Mol Biol Rev | volume = 85 | issue = 4 | pages = e0010421 | date = November 2021 | pmid = 34756086 | doi = 10.1128/MMBR.00104-21 | pmc = 8579967 | bibcode = 2021MMBR...85...21T }}</ref> and the subunit interface, implying that they are important for normal function.<ref>{{cite journal | vauthors = King TH, Liu B, McCully RR, Fournier MJ | title = Ribosome structure and activity are altered in cells lacking snoRNPs that form pseudouridines in the peptidyl transferase center | journal = Molecular Cell | volume = 11 | issue = 2 | pages = 425–35 | date = February 2003 | pmid = 12620230 | doi = 10.1016/S1097-2765(03)00040-6 | doi-access = free }}</ref>