Editing Non-coding DNA (section)

===Noncoding genes===
{{See also|Non-coding RNA}}

There are [[Gene|two types of genes]]: protein coding genes and [[Non-coding RNA|noncoding genes]].<ref>{{cite book | vauthors = Kampourakis K | date = 2017 | title = Making sense of genes | publisher = Cambridge University Press | place = Cambridge UK | isbn = 978-1-107-12813-2}}{{page needed|date=June 2022}}</ref> Noncoding genes are an important part of non-coding DNA and they include genes for [[transfer RNA]] and [[ribosomal RNA]]. These genes were discovered in the 1960s. [[Prokaryote|Prokaryotic]] genomes contain genes for a number of other noncoding RNAs but noncoding RNA genes are much more common in eukaryotes.

Typical classes of noncoding genes in eukaryotes include genes for [[small nuclear RNA]]s (snRNAs), [[small nucleolar RNA]]s (sno RNAs), [[microRNA]]s (miRNAs), [[Small interfering RNA|short interfering RNAs]] (siRNAs), [[Piwi-interacting RNA|PIWI-interacting RNAs]] (piRNAs), and [[Long non-coding RNA|long noncoding RNAs]] (lncRNAs). In addition, there are a number of unique RNA genes that produce [[Catalytic RNA|catalytic RNAs]].<ref>{{cite journal | vauthors=Cech TR, Steitz JA | title=The Noncoding RNA Revolution - Trashing Old Rules to Forge New Ones | journal=Cell|volume=157|pages=77–94|date=2014| issue=1 | doi=10.1016/j.cell.2014.03.008 | pmid=24679528 | s2cid=14852160 | doi-access=free }}</ref>

Noncoding genes account for only a few percent of prokaryotic genomes<ref>{{cite journal | vauthors = Rogozin IB, Makarova KS, Natale DA, Spiridonov AN, Tatusov RL, Wolf YI, Yin J, Koonin EV | display-authors = 6 | title = Congruent evolution of different classes of non-coding DNA in prokaryotic genomes | journal = Nucleic Acids Research | volume = 30 | issue = 19 | pages = 4264–4271 | date = October 2002 | pmid = 12364605 | pmc = 140549 | doi = 10.1093/nar/gkf549 }}</ref> but they can represent a vastly higher fraction in eukaryotic genomes.<ref>{{cite book |doi=10.1016/B978-0-12-800049-6.00171-2 |chapter=Adaptive Molecular Evolution: Detection Methods |title=Encyclopedia of Evolutionary Biology |year=2016 | vauthors = Bielawski JP, Jones C |pages=16–25 |isbn=978-0-12-800426-5 }}</ref> In humans, the noncoding genes take up at least 6% of the genome, largely because there are hundreds of copies of ribosomal RNA genes.{{citation needed|date=May 2022}} Protein-coding genes occupy about 38% of the genome; a fraction that is much higher than the coding region because genes contain large introns.{{citation needed|date=May 2022}}

The total number of noncoding genes in the human genome is controversial. Some scientists think that there are only about 5,000 noncoding genes while others believe that there may be more than 100,000 (see the article on [[Non-coding RNA]]). The difference is largely due to debate over the number of lncRNA genes.<ref>{{ cite journal | vauthors = Ponting CP, and Haerty W | date = 2022 | title = Genome-Wide Analysis of Human Long Noncoding RNAs: A Provocative Review | journal = Annual Review of Genomics and Human Genetics | volume = 23 | pages = 153–172 | doi = 10.1146/annurev-genom-112921-123710| pmid = 35395170 | s2cid = 248049706 | doi-access = free | hdl = 20.500.11820/ede40d70-b99c-42b0-a378-3b9b7b256a1b | hdl-access = free }}</ref>