Figure 1.

Epigenetic mechanisms regulating gene expression. Transcriptionally active chromatin is characterized by the presence of acetyl groups (Ac) on specific lysine residues of histones in the nucleosome, which decreases their binding to DNA, eventually leading to an open chromatin structure that permits access to transcription factors (TF). In addition, demethylation of cytidine–guanosine (CpG) sequences in the promoter region (P) of actively transcribed genes allows for the binding of transcription factors (TF). Transcriptionally inactive chromatin is characterized by histone deacetylation, promoter CpG methylation (as indicated by the presence of methyl groups, Me), and decreased binding of transcription factors. A further level of epigenetic control is provided by microRNA molecules (19–22 nucleotides in length) which bind to mRNA thus reducing the rate of translation.