**1. Introduction**

Transmission of characters in a stable, inheritable manner is governed by the genetic make-up of a cell. This information for vertical transmission of characters is carried by the macromolecule deoxyribonucleic acid (DNA). The linear sequence of nucleotides in the DNA dictates the sequence of amino acids in the proteins and hence controls all the vital processes occurring within the cell. However, the linear length of DNA molecules is very long. For example, a typical human cell contains about 2 meters long DNA. Therefore, in order to accommodate DNA into nucleus, this genetic information is contained in the form of a nucleoprotein complex called chromatin [1]. This is particularly true about eukaryotic cells. Though prokaryotic cells also contain a nucleoid, it, however, is not well-organized.

The organization of DNA into chromatin is particularly important for two main reasons.


Alongside DNA, chromatin mainly consists of small, basic positively charged group of proteins called histones. The positively charged histones bind with the negatively charged DNA in an energetically favorable manner inside chromatin [2]. These proteins have remained the focus of intensive research for many years now. Apart from DNA and histones, chromatin also contains a huge array of non-histone proteins, most of which are not as well characterized and well-studied as histones.

Earlier it was thought that compaction of DNA into chromatin solely occurs to accommodate DNA. But later it was realized that this compaction plays a paramount role in orderly organization of DNA and thereby helps in differential gene expression. The fundamental repeating unit of chromatin is the nucleosome which consists of two copies each of histones H2A, H2B, H3 and H4 wrapped around 146 bp of DNA in a left-handed helical manner [1]. The histone proteins are named in the order in which they were discovered. Because of being associated with the nucleosome core, these histone proteins are known as the core histones. Another class of histones binds DNA at the entry and exit sites into nucleosomes. This is known as the linker histone H1 and paves way for further compaction of nucleosomes into higher order chromatin structures (**Figure 1**).

Upon observation under a microscope, chromatin appears as two distinct entities within the nucleus. These are termed as euchromatin and heterochromatin. Euchromatin is the lightly stained part of chromatin which mostly lies towards the interior regions of nucleus and contains actively transcribed genomic regions. Heterochromatin is the darkly stained fraction which mostly lies towards the periphery of nucleus [3]. It contains regions which are transcriptionally silent and mostly contains repetitive DNA sequences. This spatial organization of chromatin is maintained through various mechanisms. These mechanisms serve as the "epigenetic carriers of nuclear information" within the cell and include covalent histone modifications, non-coding RNAs and chromatin remodeling complexes and lately also included DNA methylation (**Figure 2**).

### **Figure 1.**

*Representation of different levels of hierarchical chromatin organization. (A) Inside a compact chromosome, DNA and proteins are organized at different levels. (B) Ultrastructure of a nucleosome containing two copies of H2A,H2B,H3 and H4 inside 147 bp of DNA.* 

*Evolution of Epigenome as the Blueprint for Carcinogenesis DOI: http://dx.doi.org/10.5772/intechopen.97379*

**Figure 2.**

*Major players involved in the propagation of epigenetic mechanisms in cells. DNA methylation and micro RNAs are involved in gene silencing, histone modifications are involved in both silencing and expression of genes.*
