in: Epigenetics: Mechanisms and Clinical Perspectives, Yıldız Dinçer, Editor, Nova Science Publishers, New York, pp.15-31, 2016
The term epigenetics was first coined by C.H Waddington in 1942, a word derived from epigenesis and genetics. Epigenesis is the word for describing live organisms differentiating from a single cell into an organ or organism. Epigenetic mechanisms modulate gene expression patterns without affecting the deoxyribonucleic acid (DNA) sequence. To stop confusions, a consensus definition was put on use of the epigenetic trait: "stably heritable phenotype resulting from changes in a chromosome without alterations in the DNA sequence" (1). These mechanisms are acquired throughout life and depend on environmental clues such as diet, lifestyle and toxin exposure. They are heritable mechanisms so epigenetic changes can be transferred to the new cell when cells are divided with mitosis. If this process takes place in germ cells and meiosis, genomic imprinting may occur. In some occasions, such as differentiation of embryonic stem cells to specific tissues, the expression is stabilized so that the cells cannot go back to their previous state. Although all cells in one individual have the same DNA sequence, epigenetic regulation occurs at the specific gene loci in the specific cells to yield specific cellular phenotypes. A change in the phenotype does not usually effect the genotype. Gene expression can be activated or silenced via epigenetic regulations. Epigenetic changes can alter transcriptional activity of genes, and may mediate the differences in risk for certain diseases. Methylation, phosphorylation, acetylation, ubiquitination, sumoylation are included in epigenetic processes.