Today I link to and excerpt from Wikipedia‘s article on Epigenetics. Accessed4-28-2025.
All that follows is from the above resource.
By National Institutes of Health – http://commonfund.nih.gov/epigenomics/figure.aspx (rasterized from PDF), Public Domain, https://commons.wikimedia.org/w/index.php?curid=89191872
In biology, epigenetics is the study of changes in gene expression that happen without changes to the DNA sequence.[1] 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.[2] Epigenetics usually involves a change that is not erased by cell division, and affects the regulation of gene expression.[3] 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.[4] Further, non-coding RNA sequences have been shown to play a key role in the regulation of gene expression.[5] Gene expression can be controlled through the action of repressor proteins that attach to silencer regions of the DNA. These epigenetic changes may last through cell divisions 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;[6] instead, non-genetic factors cause the organism’s genes to behave (or “express themselves”) differently.[7]
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 lines of the embryo, which in turn become fully differentiated cells. In other words, as a single fertilized egg cell – the zygote – continues to 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.[8]
The term epigenesis has a generic meaning of “extra growth” that has been used in English since the 17th century.[9] In scientific publications, the term epigenetics started to appear in the 1930s (see Fig. on the right). However, its contemporary meaning emerged only in the 1990s.[10]
A definition of the concept of epigenetic trait as a “stably heritable phenotype resulting from changes in a chromosome without alterations in the DNA sequence” was formulated at a Cold Spring Harbor meeting in 2008,[11] although alternate definitions that include non-heritable traits are still being used widely.[12]
By Zephyris at the English-language Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=6998210
By MethylC5 – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=115584088
Some acetylations and some methylations of lysines (symbol K) are activation signals for transcription when present on a nucleosome, as shown in the top figure. Some methylations on lysines or arginine (R) are repression signals for transcription when present on a nucleosome, as shown in the bottom figure.
<p><a href=”https://commons.wikimedia.org/wiki/File:Histone_tails_set_for_transcriptional_repression.jpg#/media/File:Histone_tails_set_for_transcriptional_repression.jpg”><img src=”https://upload.wikimedia.org/wikipedia/commons/4/42/Histone_tails_set_for_transcriptional_repression.jpg” alt=”Histone tails set for transcriptional repression.jpg” height=”2048″ width=”2315″></a><br>By <a href=”//commons.wikimedia.org/wiki/User:MethylC5″ title=”User:MethylC5″>MethylC5</a> – <span class=”int-own-work” lang=”en”>Own work</span>, <a href=”https://creativecommons.org/licenses/by-sa/4.0″ title=”Creative Commons Attribution-Share Alike 4.0″>CC BY-SA 4.0</a>, <a href=”https://commons.wikimedia.org/w/index.php?curid=115584206″>Link</a></p>
Nucleosomes consist of four pairs of histone proteins in a tightly assembled core region plus up to 30% of each histone remaining in a loosely organized tail[33] (only one tail of each pair is shown). DNA is wrapped around the histone core proteins in chromatin. The lysines (K) are designated with a number showing their position as, for instance (K4), indicating lysine as the 4th amino acid from the amino (N) end of the tail in the histone protein. Methylations [Me], and acetylations [Ac] are common post-translational modifications on the lysines of the histone tails.
