Epigenetic

Epigenetics studies stably heritable traits (or "phenotypes") that cannot be explained by changes in DNA sequence. The Greek prefix epi- (Greek: επί- over, outside of, around) in epigenetics implies features that are "on top of" or "in addition to" the traditional genetic basis for inheritance. Epigenetics often refers to changes in a chromosome that affect gene activity and expression, but can also be used to describe any heritable phenotypic change that doesn't derive from a modification of the genome, such as Prions. Such effects on cellular and physiological phenotypic traits may result from external or environmental factors, or be part of normal developmental program. The standard definition of epigenetic requires these alterations to be heritable, either in the progeny of cells or of organisms.

The term also refers to the changes themselves: functionally relevant changes to the genome that do not involve a change in 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. 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; instead, non-genetic factors cause the organism's genes to behave (or "express themselves") differently.

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