Epigenetics

In biology, and specifically genetics, epigenetics is the study of heritable changes in gene expression or cellular phenotype caused by mechanisms other than changes in the underlying DNA sequence – hence the name epi- (Greek: επί- over, above, outer) -genetics. It refers to functionally relevant modifications to the genome that do not involve a change in the nucleotide sequence. Examples of such modifications are DNA methylation and histone modification, both of which serve to regulate gene expression without altering the underlying DNA sequence. These changes may remain through cell divisions for the remainder of the cell's life and may also last for multiple generations. However, there is no change in the underlying DNA sequence of the organism; instead, non-genetic factors cause the organism's genes to behave (or "express themselves") differently. There are objections to the use of the term epigenetic to describe chemical modification of histone since it remains unknown whether or not these modifications are heritable.

One example of epigenetic changes 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, a single fertilized egg cell – the zygote – changes into the many cell types including neurons, muscle cells, epithelium, endothelium of blood vessels, etc. as it continues to divide. It does so by activating some genes while inhibiting others.

In 2011, it was demonstrated that the methylation of mRNA has a critical role in human energy homeostasis. The obesity associated FTO gene is shown to be able to demethylate N6-methyladenosine in RNA. This opened the related field of RNA epigenetics.