DEFINITION: EPIGENOME

While genomic information is uniform in the different cells of complex organisms, the epigenome controls the differential expression of genes in specific cells. The programming of gene expression profiles is therefore dependent on the epigenome. The epigenome is composed of two modules, a component that is part of the covalent structure of DNA, methylated cytosines located in the dinucleotide sequence CG and a noncovalent module. Our understanding of the noncovalent module of the epigenome the chromatin and its associated chromatin modifying and remodeling activities is rapidly expanding in recent years (Strahl and Allis, 2000). It is now becoming clear that modifications of histones and their tails by acetylation, phosphorylation, and methylation plays an important role in determining the positioning of nucleosomes on DNA and the compactness of chromatin. Chromatin structure determines the state of activity of genes by gating the access of the transcription machinery to transcriptional regulatory regions. Chromatin structure plays a role in other genomic activities such as recombination and repai. Changes in chromatin structure play an important role in the silencing of certain genes in cancer and histone deacetylase inhibitors have demonstrated anticancer effect.