Editing Non-coding DNA (section)

===Introns===
{{Main|Intron}}

[[File:Pre-mRNA.svg|right|thumbnail|upright=1.35|Illustration of an unspliced pre-mRNA precursor, with five [[intron]]s and six [[exon]]s (top). After the introns have been removed via splicing, the mature mRNA sequence is ready for translation (bottom).]]

Introns are the parts of a gene that are transcribed into the [[precursor RNA]] sequence, but ultimately removed by [[RNA splicing]] during the processing to mature RNA. Introns are found in both types of genes: protein-coding genes and noncoding genes. They are present in prokaryotes but they are much more common in eukaryotic genomes.{{citation needed|date=June 2022}}

Group I and group II introns take up only a small percentage of the genome when they are present. Spliceosomal introns (see Figure) are only found in eukaryotes and they can represent a substantial proportion of the genome. In humans, for example, introns in protein-coding genes cover 37% of the genome. Combining that with about 1% coding sequences means that protein-coding genes occupy about 38% of the human genome. The calculations for noncoding genes are more complicated because there is considerable dispute over the total number of noncoding genes but taking only the well-defined examples means that noncoding genes occupy at least 6% of the genome.<ref>{{ cite journal | vauthors = Harrow J, Frankish A, Gonzalez JM, Tapanari E, Diekhans M, Kokocinski F, Aken BL, Barrell D, Zadissa A, Searle S | date = 2012 | title = GENCODE: the reference human genome annotation for The ENCODE Project | journal = Genome Research | volume = 22 | issue = 9 | pages = 1760–1774 | doi = 10.1101/gr.135350.111| pmid = 22955987 | pmc = 3431492 }}</ref><ref name = Piovesan>{{ cite journal | vauthors = Piovesan A, Antonaros F, Vitale L, Strippoli P, Pelleri MC, Caracausi M | date = 2019 |  title = Human protein-coding genes and gene feature statistics in 2019 | journal = BMC Research Notes | volume = 12 | issue = 1 | pages = 315 | doi = 10.1186/s13104-019-4343-8| pmid = 31164174 | pmc = 6549324 | doi-access = free }}</ref>