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{{Short description|DNA not coding for protein}}
{{split | Non-coding DNA | Junk DNA |
'''Non-coding DNA''' ('''ncDNA''') sequences are components of an organism's [[DNA]] that do not [[genetic code|encode]] [[protein]] sequences. Some non-coding DNA is [[Transcription (genetics)|transcribed]] into functional [[non-coding RNA]] molecules (e.g. [[transfer RNA]], [[microRNA]], [[Piwi-interacting RNA|piRNA]], [[ribosomal RNA]], and [[RNA interference|regulatory RNAs]]). Other functional regions of the non-coding DNA fraction include [[regulatory sequence]]s that control gene expression; [[scaffold attachment region]]s; [[origin of replication|origins of DNA replication]]; [[centromere]]s; and [[telomere]]s. Some non-coding regions appear to be mostly nonfunctional such as [[introns]], [[pseudogenes]], [[intergenic DNA]], and fragments of [[transposons]] and [[viruses]].
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==Junk DNA==
{{split | Non-coding DNA | Junk DNA |
Although many non-coding regions have biological function,<ref name="Costa non-coding3">{{cite book |title=Non-coding RNAs and Epigenetic Regulation of Gene Expression: Drivers of Natural Selection |vauthors=Costa F |date=2012 |publisher=[[Caister Academic Press]] |isbn=978-1-904455-94-3 |veditors=Morris KV |chapter=7 Non-coding RNAs, Epigenomics, and Complexity in Human Cells}}{{page needed|date=June 2022}}</ref><ref name="Nessa3">{{cite book |title=Junk DNA: A Journey Through the Dark Matter of the Genome |vauthors=Carey M |date=2015 |publisher=Columbia University Press |isbn=978-0-231-17084-0 |author-link=Nessa Carey}}{{page needed|date=June 2022}}</ref> some portion of non-coding DNA has also been described as "Junk DNA". Though exact definitions differ, this refers broadly to "any DNA sequence that does not play a functional role in development, physiology, or some other organism-level capacity."<ref name="PalazzoGregory2014" /> The term "junk DNA" was used in the 1960s.<ref name="PalazzoGregory2014" /><ref name="EhretdeHaller1963">{{cite journal | vauthors = Ehret CF, De Haller G | title = Origin, development, and maturation of organelles and organelle systems of the cell surface in Paramecium | journal = Journal of Ultrastructure Research | volume = 23 | pages = SUPPL6:1–SUPPL642 | date = October 1963 | pmid = 14073743 | doi = 10.1016/S0022-5320(63)80088-X }}</ref><ref name="Gregory Evolution Genome">{{cite book| veditors = TR |title=The Evolution of the Genome|date=2005|publisher=Elsevier |isbn=978-0-12-301463-4|pages=29–31|url=https://books.google.com/books?id=8HtPZP9VSiMC&dq=not+only+is+%22junk+dna%22+an+inappropriate+moniker&pg=PA30}}</ref> but it only became widely known in 1972 in a paper by [[Susumu Ohno]].<ref name="Ohno"/> Ohno noted that the [[mutational load]] from deleterious mutations placed an upper limit on the number of functional [[Locus (genetics)|loci]] that could be expected given a typical mutation rate. He hypothesized that mammalian genomes could not have more than 30,000 loci under selection before the "cost" from the mutational load would cause an inescapable decline in fitness, and eventually extinction.<ref name="Ohno" /> Similar calculations focusing on nucleotides rather than gene loci come to the similar conclusion that the functional portion of the human genome (given mutation rates, genome size and population size) can only be maintained up to approximately 15%.<ref>{{cite journal
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