The epigenome

We learned in Topic 8.2 that DNA is wrapped around proteins called histones. We now know that both the DNA and histones are covered in chemicals, sometimes called tags. These chemical tags form a second layer known as the epigenome. The epigenome determines the shape of the DNA-histone complex. For example it keeps genes that are inactive in a tightly packed arrangement and therefore ensures that they cannot be read (it keeps them switched off). This is known as epigenetic silencing. By contrast, it unwraps active genes so that the DNA is exposed and can easily be transcribed (switches them on). We know that the DNA code is fixed. The epigenome, however, is flexible. This is because its chemical tags respond to environmental changes. Factors like diet and stress can cause the chemical tags to adjust the wrapping and unwrapping of the DNA and so switch genes on and off. the epigenome of a cell is the accumulation of the signals it has deceived during its lifetime and it therefore acts like a cellular memory. In early development, the signals come from within the cells of the fetus and the nutrition provided by the mother 15 Important in shaping the epigenome at this stage. After birth, and throughout life, environmental factors affect the epigenome, athough signals from within the body, for example, hormones, also influence it. These factors cause the epigenome to activate or inhibit specific sets of the environmental signal stimulates proteins to carry its message inside the cell from 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 the DNA. Once attached the protein has two possible elects.