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'''Noncoding DNA''' sequences are components of an organism's [[DNA]] that do not [[genetic code|encode]] [[protein]] sequences. Some noncoding DNA is [[Transcription (genetics)|transcribed]] into functional [[non-coding RNA]] molecules (e.g. [[transfer RNA]], [[ribosomal RNA]], and [[RNA interference|regulatory RNAs]]). Other functions of noncoding DNA include the [[Transcription (genetics)|transcription]]al and [[translation (biology)|translational]] [[regulatory sequence|regulation]] of protein-coding sequences, [[scaffold attachment region]]s, [[origin of replication|origins of DNA replication]], [[centromere]]s and [[telomere]]s.
The amount of noncoding DNA varies greatly among species. Often, only a small percentage of the genome is responsible for coding proteins, but a rising percentage is being shown to have regulatory functions. When there is much non-coding DNA, a large proportion appears to have no biological function, as predicted in the 1960s. Since that time, this non-functional portion has controversially been called "junk DNA".<ref name=pennisi>{{cite journal | vauthors = Pennisi E | title = Genomics. ENCODE project writes eulogy for junk DNA | journal = Science | volume = 337 | issue = 6099 | pages = 1159–1161 | date = September 2012 | pmid = 22955811 | doi = 10.1126/science.337.6099.1159 }}</ref>gvhjvhjvcc
The international Encyclopedia of DNA Elements ([[ENCODE]]) project uncovered, by direct biochemical approaches, that at least 80% of human genomic DNA has biochemical activity.<ref name=Nature489p57>{{cite journal | author = The ENCODE Project Consortium | title = An integrated encyclopedia of DNA elements in the human genome | journal = Nature | volume = 489 | issue = 7414 | pages = 57–74 | date = September 2012 | pmid = 22955616 | pmc = 3439153 | doi = 10.1038/nature11247 | bibcode = 2012Natur.489...57T }}.</ref> Though this was not necessarily unexpected due to previous decades of research discovering many functional noncoding regions,<ref name="Costa non-coding" /><ref name=Nessa /> some scientists criticized the conclusion for conflating biochemical activity with [[biological function]].<ref name="observer">{{cite news |url=https://www.theguardian.com/science/2013/feb/24/scientists-attacked-over-junk-dna-claim |title=Scientists attacked over claim that 'junk DNA' is vital to life |author=Robin McKie|work=The Observer|date=24 February 2013 }}</ref><ref name=eddy>{{cite journal | vauthors = Eddy SR | title = The C-value paradox, junk DNA and ENCODE | journal = Current Biology | volume = 22 | issue = 21 | pages = R898–9 | date = November 2012 | pmid = 23137679 | doi = 10.1016/j.cub.2012.10.002 | author-link = Sean Eddy }}</ref><ref name=doolittle2013>{{cite journal | vauthors = Doolittle WF | title = Is junk DNA bunk? A critique of ENCODE | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 110 | issue = 14 | pages = 5294–300 | date = April 2013 | pmid = 23479647 | pmc = 3619371 | doi = 10.1073/pnas.1221376110 | authorlink = W. Ford Doolittle | bibcode = 2013PNAS..110.5294D }}</ref><ref name="PalazzoGregory2014">{{cite journal | vauthors = Palazzo AF, Gregory TR | title = The case for junk DNA | journal = PLoS Genetics | volume = 10 | issue = 5 | pages = e1004351 | date = May 2014 | pmid = 24809441 | pmc = 4014423 | doi = 10.1371/journal.pgen.1004351 }}</ref><ref name="graur">{{cite journal | vauthors = Graur D, Zheng Y, Price N, Azevedo RB, Zufall RA, Elhaik E | title = On the immortality of television sets: "function" in the human genome according to the evolution-free gospel of ENCODE | journal = Genome Biology and Evolution | volume = 5 | issue = 3 | pages = 578–90 | year = 2013 | pmid = 23431001 | pmc = 3622293 | doi = 10.1093/gbe/evt028 }}</ref> Estimates for the biologically functional fraction of the human genome based on [[comparative genomics]] range between 8 and 15%.<ref>{{cite journal | vauthors = Ponting CP, Hardison RC | title = What fraction of the human genome is functional? | journal = Genome Research | volume = 21 | issue = 11 | pages = 1769–76 | date = November 2011 | pmid = 21875934 | pmc = 3205562 | doi = 10.1101/gr.116814.110 }}</ref><ref name=kellis>{{cite journal | vauthors = Kellis M, Wold B, Snyder MP, Bernstein BE, Kundaje A, Marinov GK, Ward LD, Birney E, Crawford GE, Dekker J, Dunham I, Elnitski LL, Farnham PJ, Feingold EA, Gerstein M, Giddings MC, Gilbert DM, Gingeras TR, Green ED, Guigo R, Hubbard T, Kent J, Lieb JD, Myers RM, Pazin MJ, Ren B, Stamatoyannopoulos JA, Weng Z, White KP, Hardison RC | display-authors = 6 | title = Defining functional DNA elements in the human genome | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 111 | issue = 17 | pages = 6131–8 | date = April 2014 | pmid = 24753594 | pmc = 4035993 | doi = 10.1073/pnas.1318948111 | bibcode = 2014PNAS..111.6131K }}</ref><ref name="Rands">{{cite journal | vauthors = Rands CM, Meader S, Ponting CP, Lunter G | title = 8.2% of the Human genome is constrained: variation in rates of turnover across functional element classes in the human lineage | journal = PLoS Genetics | volume = 10 | issue = 7 | pages = e1004525 | date = July 2014 | pmid = 25057982 | pmc = 4109858 | doi = 10.1371/journal.pgen.1004525 }}</ref> However, others have argued against relying solely on estimates from comparative genomics due to its limited scope. Non-coding DNA has been found to be involved in [[epigenetic]] activity and complex [[gene regulatory network|networks of genetic interactions]] and is being explored in [[evolutionary developmental biology]].<ref name=Nessa>{{cite book|last1=Carey|first1=Nessa| authorlink =Nessa Carey|title=Junk DNA: A Journey Through the Dark Matter of the Genome|date=2015|publisher=Columbia University Press|isbn=9780231170840}}</ref><ref name=kellis /><ref name="extent functionality">{{cite journal | vauthors = Kindermann G, Leinen R | title = [Cervical polyp: What to do?] | journal = Geburtshilfe und Frauenheilkunde | volume = 33 | issue = 1 | pages = 42–5 | date = January 1973 | doi = 10.1186/1877-6566-7-2 | pmid = 4685169 | pmc = 4685169 }}</ref><ref name="Morris Epigenetics">{{cite book |editor1-last=Morris |editor1-first=Kevin | name-list-format = vanc |title=Non-Coding RNAs and Epigenetic Regulation of Gene Expression: Drivers of Natural Selection |date=2012 |publisher=Caister Academic Press |___location=Norfolk, UK |isbn=978-1904455943}}</ref>
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