Editing Exon (section)

== Contribution to genomes and size distribution ==
Although unicellular [[eukaryote]]s such as yeast have either no introns or very few, [[Animal|metazoans]] and especially [[vertebrate]] genomes have a large fraction of [[Noncoding DNA|non-coding DNA]]. For instance, in the [[human genome]] only 1.1% of the genome is spanned by exons, whereas 24% is in introns, with 75% of the genome being [[Intergenic region|intergenic DNA]].<ref>{{cite journal | author = Venter J.C.|author-link= Craig Venter| year = 2000 | title = The Sequence of the Human Genome | journal = Science | volume = 291 | issue = 5507| pages = 1304–51 | doi=10.1126/science.1058040 | pmid=11181995| bibcode = 2001Sci...291.1304V |display-authors=etal| doi-access = free }}</ref> This can provide a practical advantage in [[omics]]-aided [[health care]] (such as [[precision medicine]]) because it makes commercialized [[exome sequencing|whole exome sequencing]] a smaller and less expensive challenge than commercialized [[whole genome sequencing]]. The large variation in [[genome size]] and [[C-value]] across [[Organism|life forms]] has posed an interesting challenge called the [[C-value#Variation among species|C-value enigma]].

Across all eukaryotic genes in GenBank, there were (in 2002), on average, 5.48 exons per protein coding gene. The average exon encoded 30-36 [[amino acid]]s.<ref name="pmid11752290">{{cite journal |vauthors=Sakharkar M, Passetti F, de Souza JE, Long M, de Souza SJ |title=ExInt: an Exon Intron Database |journal=Nucleic Acids Res. |volume=30 |issue=1 |pages=191–4 |year=2002 |pmid=11752290 |pmc=99089 |doi= 10.1093/nar/30.1.191}}</ref> While the longest exon in the human genome is 11555 [[Base pair|bp]] long, several exons have been found to be only 2 bp long.<ref>{{cite journal|author1=Sakharkar M.K.|author2=Chow VT|author3=Kangueane P.|year=2004|pmid=15217358|journal=In Silico Biol|volume=4|issue=4|pages=387–93|title=Distributions of exons and introns in the human genome}}</ref> A single-nucleotide exon has been reported from the ''[[Arabidopsis thaliana|Arabidopsis]]'' genome.<ref>{{cite journal | author = Guo Lei, Liu Chun-Ming | year = 2015 | title = ''A single-nucleotide exon found in ''Arabidopsis | journal = Scientific Reports | volume = 5 | page = 18087 | doi = 10.1038/srep18087 | pmid = 26657562 | pmc = 4674806 | bibcode = 2015NatSR...518087G }}</ref> In humans, like protein coding [[mRNA]], most [[non-coding RNA]] also contain multiple exons<ref>{{cite journal |last1=Derrien |first1=T |last2=Johnson |first2=R |last3=Bussotti |first3=G |last4=Tanzer |first4=A |last5=Djebali |first5=S |last6=Tilgner |first6=H |last7=Guernec |first7=G |last8=Martin |first8=D |last9=Merkel |first9=A |last10=Knowles |first10=DG |last11=Lagarde |first11=J |last12=Veeravalli |first12=L |last13=Ruan |first13=X |last14=Ruan |first14=Y |last15=Lassmann |first15=T |last16=Carninci |first16=P |last17=Brown |first17=JB |last18=Lipovich |first18=L |last19=Gonzalez |first19=JM |last20=Thomas |first20=M |last21=Davis |first21=CA |last22=Shiekhattar |first22=R |last23=Gingeras |first23=TR |last24=Hubbard |first24=TJ |last25=Notredame |first25=C |last26=Harrow |first26=J |last27=Guigó |first27=R |title=The GENCODE v7 catalog of human long noncoding RNAs: analysis of their gene structure, evolution, and expression. |journal=Genome Research |date=September 2012 |volume=22 |issue=9 |pages=1775–89 |doi=10.1101/gr.132159.111 |pmid=22955988|pmc=3431493 }}</ref>