Epigenetics and the epigenome
Epigenetics describes heritable changes in gene function that occur without altering the base sequence of DNA.
The collection of chemical modifications associated with an organism's DNA and histone proteins is its epigenome. Unlike a change in DNA sequence, an epigenetic change alters how the existing genetic information is used.
In eukaryotic nuclei, DNA is associated with histone proteins. DNA wraps around histones to form nucleosomes, and the way these structures are organised affects how accessible a gene is to the proteins required for transcription.
Changing gene accessibility
Both DNA and histones can be chemically modified:
- methyl groups can be added to DNA;
- acetyl groups can be added to histone proteins.
These changes affect interactions between DNA and associated proteins. When DNA is held more tightly, promoter and gene regions become less accessible to transcription factors and RNA polymerase, so transcription is reduced.
When the DNA is less tightly associated with histones, the machinery required for transcription can gain access more readily.
What can alter the epigenome?
Epigenetic patterns can change during an organism's lifetime. Environmental influences include smoking, stress, exercise and diet, while signalling from within the body can also trigger modifications.
Epigenetic differences can therefore develop even between individuals with the same DNA sequence. Identical twins, for example, can become increasingly different as their epigenomes change independently with age.
Histone modifications are reversible, so patterns of gene expression can differ between cell types and can change during life.
Exam Tip: A definition of epigenetics must make both points: gene function changes, but the DNA base sequence does not.