Figure 1. Unmethylated (left) and methylated (right) cytosines.

Figure 2. Partial stability, or metastability, of epigenetic signals in cells during mitotic divisions. Methyl groups (squares) may be lost (top scenario) or de novo methylation may occur (bottom scenario), which results in different epigenotypes in cells with identical genotype, and which may lead to differential expression (wiggly lines represent mRNA molecules) of a homologous gene across the cells.

Figure 3. DNA methylation and histone acetylation act in concert in epigenetic regulation of gene expression. (a) Histone acylation and low density of ^metC in the regulatory region of a gene correlate with high transcriptional activity of a gene. (b) Histone deacylation and high density of ^metC result in compact chromatin structure, which inhibits transcription (according to Robertson KD and Wolffe AP (2000) Nature Reviews Genetics 1: 11–19.)

Figure 4. Epigenetic mechanism of discordance of monozygotic twins. Despite their identity in DNA sequences, monozygotic co-twins exhibit epigenetic differences that result in differential expression of genes and differential susceptibility to a disease in genetically identical twins.