Figure 1. Mechanisms of epigenetic gene regulation.

In its native state, DNA is wrapped around histone proteins, compacting DNA into chromatin, folded further to form chromosomes. (1) Chemical modifications catalysed by enzymes such as histone methyltransferase (HMT) or histone acetyltransferase (HAT) result in modifications on the N-terminal tails of histone proteins can control whether the chromatin is tightly packaged and silent or loosely packaged and actively expressed. (2) Another enzymatic epigenetic modification is methylation of DNA on cytosines that packages DNA into silent chromatin. (3) Small RNAs (generated by transcription from DNA or exogenously injected dsRNAs) are bound by Argonaute proteins and used as guides to find complementary mRNA sequences. The Argonaute protein can then cause gene silencing by cleaving the mRNA.