Histones 

Histones are basic proteins. Histones are enriched with basic lysine and arginine residues, Their positive charges enables  them to associate with the negatively charged DNA ( due to the phosphates , PO4 ^– ) . The histones function as spools for the thread-like DNA to wrap around. Histones may be chemically modified through the action of enzymes to regulate gene expression. Core histones share a structural motif known as the histone fold domain, formed by three α-helices connected by two loops.

Types of Histones 

Basically histones are of two category namely

Core histone  which includes  H2A, H2B, H3 & H4 and  Linker histone  which includes H1.

H2A is the core histone with the largest number of variants. The histone H2A variants which are found in most eukaryotes are H2AZ and H2AX. Phosphorylation of H2AX has been found to aid in the recruitment of proteins involved in DNA repair

Histone H2B variants are few in number and  have specialized roles in chromatin compaction during gametogenesis

Histone H3 variants include H3.3, CenHS and H3.4. H3.3  is found in transcriptionally active chromatin 

Histone H4 is the most highly conserved histone. H4 makes extensive contacts with the other three core histones in the nucleosome core particle 

Gene Organization in Eukaryotes 

The large eukaryotic genome or DNA approximately 2 m in length is packed  in to a nucleus of roughly 10 µm diameter through an orderly packaging.

Nucleosomes

Nucleosomes are the basic unit of DNA packaging in eukaryotes and represent the first level of chromatin organization. A nucleosome consist of a segment of DNA wound around a   protein core called –histone octamer. This structure is often compared to thread wrapped around a spool.

The nucleosome core particle consists of approximately 146- 147 base pairs of DNA  wrapped  around a histone octamer , consisting of 2 copies each of the core histones namely  H2A, H2B, H3, and H4.The octamer structure is a four-helix bundle comprised of two H3-H4 and two H2A-H2B dimers.

Nucleosome cores are separated by linker DNA of variable length ( 20 to 80 base pairs of linker DNA ) and are associated with the linker histone H1. Nucleosomes connected by a DNA linker of variable  forms a 10-nm beads-on-a-string array. 

Nucleosomes  pack DNA in a sequence-independent fashion. The core histones make contact with the DNA primarily through three to six hydrogen bonds between the protein main chain amides and the DNA phosphate backbone.

 Nucleosome particles can be modified in their composition, structure and location by chromatin remodeling complexes and regulates  gene expression. Nucleosomes are folded through a series of successively higher order structures to eventually form a chromosome. A nucleosome plus histone H1 is referred to as Chromatosome 

The 30 nm Fibre 

The chromatosome fold  up to a 30 nm fibre .Nucleosome core particles can stack on top of each other to form extended structures. These stacks can adopt a gentle helical configuration.  The 30 nm fibre  consists of a helical array of nucleosomes, each comprising a core particle wrapping ∼146 or 147 base pairs (bp) of DNA associated with a linker histone. Secondary structures to chromatin are demonstrated by the solenoid model and the zigzag model. The solenoid model consists of tightly wound nucleosomes containing 6 nucleosomes per turn in a regular, spiral configuration . The zigzag model is a bit looser form of chromatin with irregular configuration. In this model, nucleosomes have little face-to-face contact.

Nuclear Scaffold 

Higher order packaging consisting of loops of the 30-nm fibers attached to a proteinaceous nuclear scaffold by an interaction between the scaffold and specific DNA sequences called Scaffold/matrix attachment regions (S/MARs) . These are DNA elements that serve to compartmentalize the chromatin into structural and functional domains.

Loops are formed averaging about 300nm in length.  The 300nm fibre are compressed and folded  producing a 250 nm wide fibre .

Upon tight coiling of the 250nm wide fiber the chromatid of chromosome is formed.