The cells in a multicellular organism have nominally identical DNA sequences (and therefore the same genetic instruction sets), yet maintain different terminal phenotypes. This nongenetic cellular memory, which records developmental and environmental cues -and alternative cell states in unicellular organisms-, is the basis of epi-(above)–genetics.
Epigenetics is the study of heritable changes in gene expression (active versus inactive genes) that does not involve changes to the underlying DNA sequence — a change in phenotype without a change in genotype - which in turn affects how cells read the genes. Epigenetic change is a regular and natural occurrence but can also be influenced by several factors including age, the environment/lifestyle, and disease state. Epigenetic modifications can manifest as commonly as the manner in which cells terminally differentiate to end up as muscle cells, skin cells, liver cells, brain cells, etc. Or, epigenetic change can have more damaging effects that can result in diseases like cancer o metabolic disorders. At least three systems including DNA methylation, histone modification and non-coding RNA (ncRNA)-associated gene silencing are currently considered to initiate and sustain epigenetic change.
The approximately 40,000 genes identified by the Human Genome Project are now widely regarded as the code producing the building blocks that allow the human body to function. But in order to know where and when to act, the genes need instructions. This, in essence, is the role of epigenetics.
The DNA molecules in each cell, nearly 2 meters in length, are wound, like thread around a spool, on protein complexes containing histones. The DNA and the histones are collectively referred to as chromatin. Chromatin is unwound and opened to allow cells to use the genetic information in the DNA or compacted and closed to turn off its use and allow DNA to be stored or transported. Changes in chromatin structure are controlled by the addition or removal of chemical modifications on chromatin, and these changes help control gene expression by silencing or activating genes at the correct times and in the correct locations. Improper gene activation or silencing by loss of epigenetic control can lead to aberrant gene expression that can drive the development of diseases such as cancer, autoimmunity, diabetes, or neurological disorders.
Reference:
1.- Egger G. et al. Epigenetics in human disease and prospects for epigenetic therapy. Nature 429, 457-463 (2004)