Editing DNA (section)

=== Non-canonical bases ===

Modified bases occur in DNA. The first of these recognized was [[5-methylcytosine]], which was found in the [[genome]] of ''[[Mycobacterium tuberculosis]]'' in 1925.<ref name=Johnson1925>{{cite journal | vauthors = Johnson TB, Coghill RD | year = 1925 | title = Pyrimidines. CIII. The discovery of 5-methylcytosine in tuberculinic acid, the nucleic acid of the tubercle bacillus. | journal = Journal of the American Chemical Society | volume = 47 | pages = 2838–44 | doi=10.1021/ja01688a030|issn=0002-7863}}</ref> The reason for the presence of these noncanonical bases in bacterial viruses ([[bacteriophage]]s) is to avoid the [[restriction enzyme]]s present in bacteria. This enzyme system acts at least in part as a molecular immune system protecting bacteria from infection by viruses.<ref name="pmid27319741">{{cite journal |vauthors=Weigele P, Raleigh EA |title=Biosynthesis and Function of Modified Bases in Bacteria and Their Viruses |journal=Chemical Reviews |volume=116 |issue=20 |pages=12655–12687 |date=October 2016 |pmid=27319741 |doi=10.1021/acs.chemrev.6b00114 |doi-access=free |issn=0009-2665 }}</ref> Modifications of the bases cytosine and adenine, the more common and modified DNA bases, play vital roles in the [[epigenetics|epigenetic]] control of gene expression in plants and animals.<ref name="pmid30619465">{{cite journal |vauthors=Kumar S, Chinnusamy V, Mohapatra T |title=Epigenetics of Modified DNA Bases: 5-Methylcytosine and Beyond |journal=Frontiers in Genetics |volume=9 |pages=640 |date=2018 |pmid=30619465 |pmc=6305559 |doi=10.3389/fgene.2018.00640 |issn=1664-8021 |doi-access=free }}</ref>

A number of noncanonical bases are known to occur in DNA.<ref name="pmid28941008">{{cite journal | vauthors = Carell T, Kurz MQ, Müller M, Rossa M, Spada F | title = Non-canonical Bases in the Genome: The Regulatory Information Layer in DNA | journal = Angewandte Chemie | volume = 57 | issue = 16 | pages = 4296–4312 | date = April 2018 | pmid = 28941008 | doi = 10.1002/anie.201708228 }}</ref> Most of these are modifications of the canonical bases plus uracil.

* Modified '''Adenine'''
** N6-carbamoyl-methyladenine
** N6-methyadenine
* Modified '''Guanine'''
** 7-Deazaguanine
** 7-Methylguanine
* Modified '''Cytosine'''
** N4-Methylcytosine
** 5-Carboxylcytosine
** 5-Formylcytosine
** 5-Glycosylhydroxymethylcytosine
** 5-Hydroxycytosine
** 5-Methylcytosine
* Modified '''Thymidine'''
** α-Glutamythymidine
** α-Putrescinylthymine
* '''Uracil''' and modifications
** [[Base J]]
** Uracil
** 5-Dihydroxypentauracil
** 5-Hydroxymethyldeoxyuracil
* Others
** Deoxyarchaeosine
** 2,6-Diaminopurine (2-Aminoadenine)