Non-coding DNA: Difference between revisions

Revision as of 02:47, 18 March 2023

Revision as of 13:28, 26 March 2023

Line 64:

===Centromeres===

{{Main|Centromere}}

[[File:Human karyotype with bands and sub-bands.png|thumb|Schematic [[karyotype|karyogram]] of a human, showing an overview of the [[human genome]] on [[G banding]], wherein non-coding DNA is present at the centromeres (shown as narrow segment of each chromosome), and also occurs to a greater extent in darker ([[GC-content|GC poor]]) regions.<ref name=Romiguier2017>{{cite journal | vauthors = Romiguier J, Roux C | title = Analytical Biases Associated with GC-Content in Molecular Evolution | journal = Frontiers in Genetics | volume = 8 | issue = | pages = 16 | year = 2017 | pmid = 28261263 | pmc = 5309256 | doi = 10.3389/fgene.2017.00016 | doi-access = free }} </ref><br>]]

Centromeres are the sites where spindle fibers attach to newly replicated chromosomes in order to segregate them into daughter cells when the cell divides. Each eukaryotic chromosome has a single functional centromere that's seen as a constricted region in a condensed metaphase chromosome. Centromeric DNA consists of a number of repetitive DNA sequences that often take up a significant fraction of the genome because each centromere can be millions of base pairs in length. In humans, for example, the sequences of all 24 centromeres have been determined<ref>{{ cite journal | vauthors = Altemose N, Logsdon GA, Bzikadze AV, Sidhwani P, Langley SA, Caldas GV, et al. | title = Complete genomic and epigenetic maps of human centromeres | journal = Science | volume = 376 | pages = 56 | date = 2021 | issue = 6588 | doi = 10.1126/science.abl4178| pmid = 35357911 | pmc = 9233505 | s2cid = 247853627 }}</ref> and they account for about 6% of the genome. However, it's unlikely that all of this noncoding DNA is essential since there is considerable variation in the total amount of centromeric DNA in different individuals.<ref>{{cite journal | vauthors = Miga KH | title = Centromeric satellite DNAs: hidden sequence variation in the human population | journal = Genes | volume = 10 | pages = 353 | date = 2019 | issue = 5 | doi = 10.3390/genes10050352| pmid = 31072070 | pmc = 6562703 | doi-access = free }}</ref> Centromeres are another example of functional noncoding DNA sequences that have been known for almost half a century and it's likely that they are more abundant than coding DNA.

Line 108:

===Junk DNA===

{{Main|Junk DNA}}

Junk DNA is DNA that has no biologically relevant function such as pseudogenes and fragments of once active transposons. Bacteria and viral genomes have very little junk DNA<ref>{{cite journal | vauthors = Gil R, and Latorre A | date = 2012 | title = Factors behind junk DNA in bacteria | journal = Genes | volume = 3 | issue = 4 | pages = 634-650634–650 | doi = 10.3390/genes3040634 | pmid = 24705080 | pmc = 3899985 | doi-access = free }}</ref><ref>{{Cite journal |lastlast1=Brandes |firstfirst1=Nadav |last2=Linial |first2=Michal |date=2016 |title=Gene overlapping and size constraints in the viral world |url=http://biologydirect.biomedcentral.com/articles/10.1186/s13062-016-0128-3 |journal=Biology Direct |language=en |volume=11 |issue=1 |pages=26 |doi=10.1186/s13062-016-0128-3 |pmid=27209091 |pmc=4875738 |issn=1745-6150}}</ref> but some eukaryotic genomes may have a substantial amount of junk DNA.<ref name="PalazzoGregory2014">{{cite journal | vauthors = Palazzo AF, Gregory TR | title = The case for junk DNA | journal = PLoSPLOS Genetics | volume = 10 | issue = 5 | pages = e1004351 | date = May 2014 | pmid = 24809441 | pmc = 4014423 | doi = 10.1371/journal.pgen.1004351 }}</ref> The exact amount of nonfunctional DNA in humans and other species with large genomes has not been determined and there is considerable controversy in the scientific literature.<ref>{{cite journal | last = Morange | first = Michel | date = 2014 | title = Genome as a Multipurpose Structure Built by Evolution | journal = Perspectives in Biology and Medicine | volume = 57 | issue = 1 | pages = 162-171162–171 | doi = 10.1353/pbm.2014.0008 | pmid = 25345709 | s2cid = 27613442 | url = https://hal.archives-ouvertes.fr/hal-01480552/file/ARTICLE%20ENCODE%20MM%2070114%20corrige%C2%A6%C3%BC.pdf }}</ref><ref>{{cite journal | vauthors = Haerty W, and Ponting CP | title = No Gene in the Genome Makes Sense Except in the Light of Evolution. | year = 2014 | journal = Annual Review of Genomics and Human Genetics | volume =25 | pages = 71-9271–92 | doi = 10.1146/annurev-genom-090413-025621| pmid = 24773316 }}</ref>

The nonfunctional DNA in bacterial genomes is mostly located in the intergenic fraction of non-coding DNA but in eukaryotic genomes it may also be found within [[introns]]. It's important to note that there are many examples of functional DNA elements in non-coding DNA (see above) and there are no scientists who claim that all non-coding DNA is junk.