Figure 1.

Epigenetic mechanisms include DNA methylation, histone modifications, and miRNAs. DNA methylation results from a balance between unmethylated cytosines, methylated cytosines (5^mC), and hydroxymethylatedcytosines (5^hmC) in CpG dinucleotides, catalyzed by DNMTs (C→5^mC) and TETs (5^mC→5^hmC→C). Methylation leads to structural changes of chromatin and is associated with gene silencing. The basic unit of chromatin is the nucleosome, a nucleoprotein complex that consists of DNA wrapped around an octameric core of histones. N-terminal tails of core histones are post-translationally modified mostly by acetylation, methylation, phosphorylation, and ubiquitination (green and yellow pentagons). Histone modifications can alter DNA accessibility, facilitate RNA polymerase activity and can increase or decrease gene transcriptional activity. MiRNAs bind to mRNA and cause RNA degradation, thereby limiting the production of gene products and leading to gene silencing. Abbreviations: miRNA: micro-RNA; mRNA: messenger RNA.