Editing Epigenetics (section)

{{Short description|Study of DNA modifications that do not change its sequence}}
{{other uses|Epigenetic (disambiguation)}}
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[[File:Epigenetic mechanisms.png|thumb|Epigenetic mechanisms]]

In biology, '''epigenetics''' is the study of changes in [[gene expression]] that happen without changes to the DNA sequence.<ref name="Epigenetics 2009 review">{{cite journal | vauthors = Dupont C, Armant DR, Brenner CA | title = Epigenetics: definition, mechanisms and clinical perspective | journal = Seminars in Reproductive Medicine | volume = 27 | issue = 5 | pages = 351–7 | date = September 2009 | pmid = 19711245 | pmc = 2791696 | doi = 10.1055/s-0029-1237423 | quote = In the original sense of this definition, epigenetics referred to all molecular pathways modulating the expression of a genotype into a particular phenotype. Over the following years, with the rapid growth of genetics, the meaning of the word has gradually narrowed. Epigenetics has been defined and today is generally accepted as 'the study of changes in gene function that are mitotically and/or meiotically heritable and that do not entail a change in DNA sequence.' }}</ref> The Greek prefix ''epi-'' (ἐπι- "over, outside of, around") in ''epigenetics'' implies features that are "on top of" or "in addition to" the traditional (DNA sequence based) genetic mechanism of inheritance.<ref name = science>{{cite news | title=Beware the pseudo gene genies | vauthors = Rutherford A | url= https://www.theguardian.com/science/2015/jul/19/epigenetics-dna--darwin-adam-rutherford | work=[[The Guardian]] | date=19 July 2015 }}</ref> Epigenetics usually involves a change that is not erased by cell division, and affects the regulation of gene expression.<ref>{{cite journal | vauthors = Deans C, Maggert KA | title = What do you mean, "epigenetic"? | journal = Genetics | volume = 199 | issue = 4 | pages = 887–896 | date = April 2015 | pmid = 25855649 | doi = 10.1534/genetics.114.173492 | pmc = 4391566 }}</ref> Such effects on cellular and physiological traits may result from environmental factors, or be part of normal development. 

The term also refers to the mechanism of changes: functionally relevant alterations to the [[genome]] that do not involve mutation of the [[nucleotide sequence]]. Examples of mechanisms that produce such changes are [[DNA methylation]] and [[histone modification]], each of which alters how genes are expressed without altering the underlying [[DNA]] sequence.<ref>{{cite journal | vauthors = Kanwal R, Gupta S | title = Epigenetic modifications in cancer | journal = Clinical Genetics | volume = 81 | issue = 4 | pages = 303–311 | date = April 2012 | pmid = 22082348 | pmc = 3590802 | doi = 10.1111/j.1399-0004.2011.01809.x }}</ref> Further, non-coding RNA sequences have been shown to play a key role in the regulation of gene expression.<ref>{{cite journal | vauthors = Frías-Lasserre D, Villagra CA | title = The Importance of ncRNAs as Epigenetic Mechanisms in Phenotypic Variation and Organic Evolution | journal = Frontiers in Microbiology | volume = 8 | pages = 2483 | date = 2017 | pmid = 29312192 | pmc = 5744636 | doi = 10.3389/fmicb.2017.02483 | doi-access = free }}</ref> Gene expression can be controlled through the action of [[repressor protein]]s that attach to [[silencer (DNA)|silencer]] regions of the DNA. These epigenetic changes may last through [[cell division]]s for the duration of the cell's life, and may also last for multiple generations, even though they do not involve changes in the underlying DNA sequence of the organism;<ref name="pmid17522671">{{cite journal | vauthors = Bird A | title = Perceptions of epigenetics | journal = Nature | volume = 447 | issue = 7143 | pages = 396–398 | date = May 2007 | pmid = 17522671 | doi = 10.1038/nature05913 | s2cid = 4357965 | doi-access = free | bibcode = 2007Natur.447..396B }}</ref> instead, non-genetic factors cause the organism's genes to behave (or "express themselves") differently.<ref>{{cite web| vauthors = Hunter P |date=1 May 2008|title=What genes remember|url=https://www.prospectmagazine.co.uk/magazine/whatgenesremember|url-status=dead|archive-url=https://web.archive.org/web/20080501094940/http://www.prospect-magazine.co.uk/article_details.php?id=10140|archive-date=1 May 2008|access-date=26 July 2012 |magazine=Prospect Magazine|issue=146}}</ref>

One example of an epigenetic change in [[eukaryotic]] biology is the process of [[cellular differentiation]]. During [[morphogenesis]], [[totipotent]] [[stem cells]] become the various [[pluripotent]] [[cell line]]s of the [[embryo]], which in turn become fully differentiated cells. In other words, as a single fertilized [[egg cell]] – the [[zygote]] – continues to [[mitosis|divide]], the resulting daughter cells change into all the different cell types in an organism, including [[neurons]], [[muscle cells]], [[epithelium]], [[endothelium]] of [[blood vessels]], etc., by activating some genes while inhibiting the expression of others.<ref name="pmid17522676">{{cite journal | vauthors = Reik W | title = Stability and flexibility of epigenetic gene regulation in mammalian development | journal = Nature | volume = 447 | issue = 7143 | pages = 425–32 | date = May 2007 | pmid = 17522676 | doi = 10.1038/nature05918 | bibcode = 2007Natur.447..425R | s2cid = 11794102 }}</ref>