Figure 1

Heterochromatin is the compacted "closed" form of chromatin associated with gene silencing. Activation of chromatin to its more "open" form which allows gene expression to occur is regulated by modification of core histones by specific co-activator complexes containing enzymes which can acetylate, phosphorylate or methylate histone tails. Removal of the linker histone H1 and changes in DNA methylation state are also important in this process. This is reversed by corepressor complexes that include histone deacetylases (HDACs) and both DNA and histone methylases, thereby causing gene silencing.