Epigenetics

DNA sequence is a critical source of information for all biological processes, but the regulation of gene activity is necessary for this genetic potential to be utilized. Epigenetics refers to molecular mechanisms that contribute to the regulation of gene activity, resulting in either increases or decreases in gene expression (transcription). These mechanisms do not involve modifications to the DNA sequence. In 1942, Conrad Waddington coined the term epigenetics to describe the complex interplay between genes and their products, which occur during development to establish the divergence of phenotypes (characteristics) that emerge from a given genotype. An example of the role of epigenetics in development is illustrated by the process of cellular differentiation. An organism has a single genome, and that genome is copied when cells divide, such that each cell within an organism has the same sequence of DNA. However, there are hundreds of different cell types that emerge across development within an organism, such as neurons, muscle cells, and immune cells, which differ from each other in both function and form. This divergence in phenotype of cells is established and maintained through epigenetic modifications.

There are currently three broad types of epigenetic modifications: (a) chemical modifications to cytosine within the DNA sequence (e.g., DNA methylation), (b) posttranslational histone protein chemical modifications (e.g., histone acetylation, methylation), and (c) expression noncoding RNAs. Histones are proteins around which DNA is wrapped within the cell nucleus, and noncoding RNA refers to RNA molecules that are created through transcription but that do not undergo translation to become a protein. Although each of these types of epigenetic mechanisms alters gene activity via different processes, the collective action of these mechanisms is to alter the levels of gene transcription, in some cases resulting in gene silencing. The study of epigenetic mechanisms has been integrated into the biological sciences, neuroscience, and medicine. Epigenetics may also provide insights into the interplay between genes and environments during the life span of an individual and in the transmission across generations of the effects of environmental exposures (i.e., nutritional, stress, toxins) and social experiences. This entry discusses this interplay and generational transmission, which have broad implications within the social and behavioral sciences, particularly for the study of human development.