The epigenome

• DNA-histone complexes are covered in chemical 'tags', forming a second layer known as the epigenome.
• The epigenome determines the shape of a DNA-histone complex. For example, the epigenome can cause epigenetic silencing. This occurs when inactive genes are packed tightly in such a way that they cannot be read, hence keeping the genes switched off.
• The epigenome also has a role in unwrapping active genes so that the DNA is exposed and can easily be transcribed, hence allowing the genes to be switched on.
• The DNA code is fixed, however the epigenome is flexible. This is because the chemical tags respond to environmental changes, adjusting the wrapping/unwrapping of DNA to switch genes off and on.
• The epigenome of a cell acts a as a 'cellular memory'. During early fetal development, for example, the nutrition provided by the mother will help shape the epigenome. However, after birth, the epigenome is largely determined by the external environment.
• An environmental signal will stimulate proteins to carry its message inside the cell, where it is passed by a series of other proteins into the nucleus. Here, the message passes to a specific protein which can be attached to a specific sequence of bases on DNA.
• Once a specific sequence of bases has been attached to the specific protein inside the nucleus, the attached protein has two possible effects : methylation or acetylation.
• Acetylation of histones can lead to the activation or inhibition of a gene.
• Methylation of DNA occurs by attracting enzymes that can add or remove methyl groups.