**2. Epigenetics and chromatin biology: unifying themes and differences**

At its heart, epigenetics refers to the study of heritable changes in gene expression without changes in the DNA sequence. This term was coined by Waddington and as the name indicates, epi (above or over genetics) is any moiety that can be stably inherited by cells across many generations without altering the sequence of nucleotides in the DNA. The study of epigenetics previously involved study of covalent histone modifications and non-coding RNAs. However, DNA methylation has also been increasingly recognized as an epigenetic phenomenon owing to its non-sequence based heritable nature and its importance in maintaining cellular homeostasis and association of its perturbations with various diseases. Therefore, the definition and scope of epigenetics has changed dynamically since the inception of the field.

Quite often, epigenetics and chromatin biology are very loosely stated terms. However, to be more precise, epigenetics refers to the study of "epigenetic marks or signatures" which play a prominent role in maintenance of cellular homeostasis whereas chromatin biology refers to the study of "chromatin structure and function". This encompasses nuclear dynamics, topology, localisation, organisation and three-dimensional (3D) structure [3]. There is a huge overlap between the two terms, and these are often used interchangeably. For example, epigenetic signatures and modifications play a paramount role in the maintenance of nuclear topology, overall chromatin organization and chromatin states.

Field of epigenetics is very interesting because of the reversible nature of epigenetic changes. This means that although these changes can be stably inherited, however, unlike DNA sequence, these changes can also be reversed under particular conditions. In fact, mechanisms are well in place within the cells which lead to the reversal of these modifications [4]. Interestingly, these changes can also be targeted for the reversal externally, using specific enzymes, under desired conditions. This may include the reversal of epigenetic modifications involved in disease progression with the help of enzymes [5]. For example, reversal of an epigenetic modification that is involved in carcinogenesis by an enzyme specific for the reversal to alleviate some of the symptoms.

Epigenetic modifications play a very prominent role in almost all the cellular processes like growth, cell division, maintenance of cellular identity etc. Therefore, any changes in these modifications can lead to serious outcomes. Perturbations in epigenetic modifications have been observed to be involved in various deleterious conditions including cancer [6].

In this chapter, we shall discuss about the various epigenetic mechanisms, their importance, major functions that they carry out in the cells and changes to these marks and their implications in cancer.
