THEORETICAL PAPERS AND REVIEWS
What Is Epigenetics
L. I. Korochkin
Institute of Gene Biology, Russian Academy of Sciences, Moscow, 119334 Russia Kol’tsov Institute ofDevelopmental Biology, Russian Academy of Sciences, 119991 Russia; e-mail: email@example.com
Received February 15, 2006
Abstract—The review gives a deﬁnition of epigenetics, considers its history, and describes the relevant phenomena. It is emphasized that epigenetic events agree with the current genetic paradigm, rather than striking its foundation. DOI: 10.1134/S102279540609002X
DEFINITIONOF EPIGENETICS Epigenetic has been much spoken about, often without a clear understanding of the meaning of the term. Most authors did not bother to give a deﬁnition of epigenetics, which led to a vague notion of epigenetics and adjacent disciplines. In many cases the term epigenetics has been used as a plaster for all sores, as if it eliminates all contradictions and solves all problems ofbiology, developmental genetics, physiology, and even the theory of evolution! There is a silent agreement that everything becomes clear and conceivable when this magic word is just mentioned. Epigenetics is regarded as something new, something that has been unknown until recently and has just arose to resolve all debates, completely reform modern genetics, and call for a revision of the entirebiological paradigm. It proved rather difﬁcult to deﬁne epigenetics and epigenetic processes. Here is an example of an erroneous deﬁnition: Epigentics is a mechanism whereby genes form the phenotype . In classical genetic works, epigenetics is deﬁned as a branch of biology that focuses on causal analysis of the development . This deﬁnition is again incorrect, equating developmental mechanics(experimental and developmental biology) with epigenetics. Yet the scope of developmental mechanics is broader, including processes that do not directly depend on the genes. The same authors have provided a more appropriate deﬁnition of epigenetic processes as “all processes relevant to expression and interactions of genetic material. Epigenetic mechanisms act at three levels of cell organization. (1)They directly regulate the gene function (switching it on or off) or modulate synthesis of speciﬁc proteins. (2) They regulate cell differentiation by modifying RNA translation into proteins. (3) They regulate the topographic distribution and functions of proteins.” To supplement, genes are involved in regulating the development at the level of intercellular interactions and at the organismic level.In addition, Rieger et al.  introduced the term epigenotype as a complex of interacting genetic and nongenetic factors that determine a speciﬁc epiphenotype. In 1965, I proposed the equivalent terms cytogenotype and cytophenotype . ORIGIN OF EPIGENETICS It was T.H. Morgan who identiﬁed the range of phenomena that are known as epigenetic now and described them in general detail as earlyas in the 1920s and 1930s. He, one of the founders of both genetics and developmental mechanics, bridged the abyss that existed at that time between genetics and experimental embryology. It was Morgan who formulated the question of how cells with identical gene sets are differently specialized during ontogeny. Answering this question, he wrote: “it is possible that, as an organism develops, itsgenes also grow more and more complex or change somehow according to the protoplasm region where they belong and that such changes reciprocally affect the protoplasm” . In other words, different genes function in different regions of a developing embryo, because such regions differ in the cytoplasm according to which genes “change somehow.” This circumstance was also noted by Morgan: “The...