Figure 2.

Fundamental epigenetic mechanisms. Epigenetic mechanisms refer to three highly interrelated mechanisms that regulate gene expression, as follows: (1) DNA methylation refers to the covalent addition of a methyl group to the 5-position of cytosine in the context of CpG dinucleotides in mammals; (2) histone density and post-translational modifications affect chromatin structure: the fundamental repeating unit of chromatin is the nucleosome that is comprised of an octamer of two each of four core histone proteins; and (3) nuclear RNA-based mechanisms including long noncoding RNAs can direct chromatin structure. Reprinted with permission from Yan et al.⁴³