Epigenetics is the study of heritable changes in the phenotype of a cell or organism that are not caused by its genotype. The molecular basis of an epigenetic profile arises from covalent modifications of protein and DNA components of chromatin. The epigenetic profile of a cell often dictates cell fate, as well as mammalian development, aging and disease. Epigenetic changes may persist for the remainder of a cell's life, but may also last for multiple generations in a lineage. Here, we provide an overview of the molecular basis for epigenetics and methods for studying DNA methylation.
In eukaryotes, chromatin is organized into nucleosome core particles that consist of approximately 147 bp of DNA and an octamer of histones (typically, two each of the core histones: H2A, H2B, H3 and H4) (1). The linker histone, H1, can further condense chromatin by binding to linker DNA between the nucleosome core particles. In mammals, chromatin can be generally classified as condensed, transcriptionally silent heterochromatin, or less-condensed, transcriptionally active euchromatin. Most genomic DNA is heterochromatin, which constitutes telomeres, pericentric regions and areas rich in repetitive sequences.
The core histones consist of a globular C-terminal domain and an unstructured N-terminal tail. Although a variety of modifications occur throughout the histone protein (Table 1), they occur primarily on the N-terminal tail. Some of these changes are enzymatically reversible. In general, the biological significance of all these modifications is not well understood, but the modifications are known to influence transcription, DNA repair, DNA replication and chromatin condensation. A "histone code" hypothesis is being tested by researchers to determine if combinations of histone modifications can be used to predict changes in gene expression (2-3). A comprehensive list of histone-modifying enzymes can be found in a review of mammalian epigenetic mechanisms by Kim et al (4). New England Biolabs offers a selection of unmodified, recombinant human histones
(NEB#E5350) modifying enzymes (see product list).