Editing Caenorhabditis elegans (section)

====Size and gene content====
The ''C. elegans'' genome is about 100 million [[base pair]]s long and consists of six pairs of chromosomes in hermaphrodites or five pairs of autosomes with XO chromosome in male ''C. elegans'' and a [[mitochondrial genome]]. Its [[gene density]] is about one gene per five [[kilo-base pair]]s. [[Intron]]s make up 26% and [[intergenic region]]s 47% of the genome. Many genes are arranged in clusters and how many of these are [[operon]]s is unclear.<ref>{{cite journal | vauthors = Blumenthal T, Evans D, Link CD, Guffanti A, Lawson D, Thierry-Mieg J, Thierry-Mieg D, Chiu WL, Duke K, Kiraly M, Kim SK | title = A global analysis of ''Caenorhabditis elegans'' operons | journal = Nature | volume = 417 | issue = 6891 | pages = 851–4 | date = June 2002 | pmid = 12075352 | doi = 10.1038/nature00831 | bibcode = 2002Natur.417..851B | s2cid = 4351788 }}</ref> ''C. elegans'' and other nematodes are among the few eukaryotes currently known to have operons; these include [[trypanosomes]], [[flatworm]]s (notably the [[trematode]] ''[[Schistosoma mansoni]]''), and a primitive [[chordate]] [[tunicate]] ''[[Oikopleura dioica]]''. Many more organisms are likely to be shown to have these operons.<ref>{{cite journal | vauthors = Blumenthal T | title = Operons in eukaryotes | journal = Briefings in Functional Genomics & Proteomics | volume = 3 | issue = 3 | pages = 199–211 | date = November 2004 | pmid = 15642184 | doi = 10.1093/bfgp/3.3.199 | doi-access = free }}</ref>

The genome contains an estimated 20,470 [[protein]]-coding [[gene]]s.<ref>{{cite web
 |date=10 August 2011
 |title=WS227 Release Letter
 |url=http://www.wormbase.org/wiki/index.php/WS227
 |publisher=[[WormBase]]
 |access-date=2013-11-19
 |archive-url=https://archive.today/20131128221823/http://www.wormbase.org/wiki/index.php/WS227
 |archive-date=28 November 2013
 |url-status=dead
 }}</ref> About 35% of ''C. elegans'' genes have human [[Homology (biology)|homologs]]. Remarkably, human genes have been shown repeatedly to replace their ''C. elegans'' homologs when introduced into ''C. elegans''. Conversely, many ''C. elegans'' genes can function similarly to mammalian genes.<ref name="Introduction to C. Elegans" />

The number of known [[RNA gene]]s in the genome has increased greatly due to the 2006 discovery of a new class called ''[[21U-RNA]]'' genes,<ref>{{cite journal | vauthors = Ruby JG, Jan C, Player C, Axtell MJ, Lee W, Nusbaum C, Ge H, Bartel DP | title = Large-scale sequencing reveals 21U-RNAs and additional microRNAs and endogenous siRNAs in ''C. elegans'' | journal = Cell | volume = 127 | issue = 6 | pages = 1193–207 | date = December 2006 | pmid = 17174894 | doi = 10.1016/j.cell.2006.10.040 | doi-access = free }}</ref> and the genome is now believed to contain more than 16,000 RNA genes, up from as few as 1,300 in 2005.<ref>{{cite journal | vauthors = Stricklin SL, Griffiths-Jones S, Eddy SR | title = ''C. elegans'' noncoding RNA genes | journal = WormBook | pages = 1–7 | date = June 2005 | pmid = 18023116 | pmc = 4781554 | doi = 10.1895/wormbook.1.1.1 }}</ref>

Scientific curators continue to appraise the set of known genes; new gene models continue to be added and incorrect ones modified or removed.

The reference ''C. elegans'' genome sequence continues to change as new evidence reveals errors in the original sequencing. Most changes are minor, adding or removing only a few base pairs of DNA. For example, the WS202 release of WormBase (April 2009) added two base pairs to the genome sequence.<ref name="WS202">{{cite web
 |date=29 May 2009
 |title=WS202 Release Letter
 |url=http://www.wormbase.org/wiki/index.php/WS202
 |archive-url=https://archive.today/20131201022934/http://www.wormbase.org/wiki/index.php/WS202
 |url-status=dead
 |archive-date=December 1, 2013
 |publisher=[[WormBase]]
 |access-date=2013-11-19
 }}</ref> Sometimes, more extensive changes are made as noted in the WS197 release of December 2008, which added a region of over 4,300 bp to the sequence.<ref name="WS197">{{cite web
 |date=27 November 2008
 |title=WS197 Release Letter
 |url=http://www.wormbase.org/wiki/index.php/WS197
 |publisher=[[WormBase]]
 |access-date=2013-11-19
 |archive-url=https://web.archive.org/web/20191017232138/https://wiki.wormbase.org/index.php/WS197
 |archive-date=17 October 2019
 |url-status=dead
 }}</ref><ref name="GenomeChanges">{{cite web
 |date=15 June 2011
 |title=Genome sequence changes
 |url=http://www.wormbase.org/wiki/index.php/Genome_sequence_changes
 |access-date=2011-08-13
 |publisher=[[WormBase]]
 |archive-url=https://web.archive.org/web/20191017232140/https://wiki.wormbase.org/index.php/Genome_sequence_changes
 |archive-date=17 October 2019
 |url-status=dead
 }}</ref>

The ''C. elegans'' Genome Project's Wilson et al. 1994 found [[CelVav]]<ref name="Bustelo-2000" /> and a [[von Willebrand factor A]] domain<ref name="Sadler-1998" /> and with Wilson et al. 1998 provides the first credible evidence for an [[aryl hydrocarbon receptor]] (AHR) [[homologous gene|homolog]] outside of vertebrates.<ref name="Hahn-2002" /> 2