Epigenetics is the study of heritable changes in gene function that happen without a change in the base sequence of DNA. Chemical tags on DNA and on the histone proteins it is wrapped around decide whether genes can be read. The full set of these tags in a cell is called its epigenome.Epigenetic control is central to development. As cells of the embryo differentiate, epigenetic marks switch off genes that the cell will never need and keep on the genes it does need. This locks each cell into its identity, which is why a liver cell does not suddenly start behaving like a neurone. These marks are copied when a cell divides by mitosis, so daughter cells keep the same cell type. Epigenetic marking is also one reason cells lose potency as they specialise.Two well-known examples in mammals:
• X-inactivation: in females, one of the two X chromosomes in each cell is permanently switched off early in development, so females do not make twice as much of X-linked gene products.
• Genomic imprinting: some genes are expressed only from the copy inherited from one particular parent, because the other copy is epigenetically silenced.Unlike the base sequence, the epigenome can respond to the environment. Children conceived during the Dutch Hunger Winter famine of 1944 to 1945 had altered epigenetic marks and higher rates of some diseases decades later, showing that the maternal diet can influence development through epigenetics. Some marks may even be passed on to the next generation.
• X-inactivation: in females, one of the two X chromosomes in each cell is permanently switched off early in development, so females do not make twice as much of X-linked gene products.
• Genomic imprinting: some genes are expressed only from the copy inherited from one particular parent, because the other copy is epigenetically silenced.Unlike the base sequence, the epigenome can respond to the environment. Children conceived during the Dutch Hunger Winter famine of 1944 to 1945 had altered epigenetic marks and higher rates of some diseases decades later, showing that the maternal diet can influence development through epigenetics. Some marks may even be passed on to the next generation.