Epigenetics is the study of the external modifications that DNA undergoes to express genes. The external modifications do not change the sequence of DNA, but instead they affect how the cells controlled by the DNA read the expressed genes.
Epigenetic changes do affect the physical structure of DNA without affecting the sequence of the base pairs in the compound. These types of changes may add or remove chemical elements to affect how certain genes are expressed. Epigenetic alterations could also modify histone compounds. Histones are proteins that encompass DNA molecules. DNA strands wrap themselves around histones in order to shrink in size. Epigentic alterations can cause histones to relax and DNA to become more accessible for enzymes and proteins to read and express certain genes.