Albrecht Kossel discovered histones in 1884. The word “histone” is derived from the German word “histon”.

The nuclear DNA in all eukaryotic cells is highly condensed due to its association with histones, proteins that counterbalance their electrostatic charge. These proteins are abundant in arginine and lysine, which are commonly found in eukaryotic cells.

Thus, Histones are a special group of proteins seen in the nuclei of eukaryotic cells which play a role in the formation of chromatin and DNA folding. These proteins are highly alkaline and possess a positive charge.

Histones act as spools, allowing DNA to spiral around them, forming nucleosomes–which look like beads on a string. These nucleosomes are then bound together into 30-nanometer fibres, forming compacted chromatin. By restricting the DNA from becoming twisted and tangled, histones protect it from any damage.

Additionally, these histones are involved in DNA replication and the regulation of genes. In their absence in the chromosomes, the unwound DNA would be too lengthy. For instance, in every cell of humans, there are approximately 1.8 meters of DNA if it is stretched out completely, but when it is twisted about the histones, its length decreases to approximately 0.09 mm of the 30 nm diameter chromatin fibres.

Classes of Histones

There are two primary classes of Histones:

Core Histones: Families included are H2A, H2B, H3, and H4.

Linker Histones: Families included are - H1 and H5.

Two of each of these core histone proteins assemble to form an octameric nucleosome core particle, while 147 base pairs of DNA surround their substance.

The H1 linker histone proteins associate the nucleosomes at the start and end sites of DNA, thus securing DNA in place and aiding in the formation of higher-order structures.

What is a Histone Octamer?

A complex of eight positively charged histone proteins, known as a histone octamer, helps with DNA packaging. It is located at the centre of the nucleosome core particle and consists of two copies of each of the four core histone proteins: H2A, H2B, H3, and H4. This octamer is formed when a tetramer of two copies of H3 and H4 combines with two dimers of H2A/H2B.

Negatively charged DNA surrounds the positively charged histone octamer to form nucleosomes, assisting in the nucleosome-formation.

Each of the histones has a C-terminal histone-fold and an N-terminal tail, both of which are involved in interactions with the DNA via a series of weak interactions, such as salt bridges and hydrogen bonds. This association keeps the histone octamer and DNA loosely connected, allowing them to easily separate and rearrange.

Histone Octamer Diagram

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Histone Octamer Structure

The structure of the octameric histone core of nucleosomes was observed under X-ray crystallography to a 3.1 A resolution. This histone octamer is a tripartite assembly wherein a tetramer is centrally placed and flanked by dimers - H2A-H2B.

The linked histone HI associates with each nucleosome, forming a 10 nm fibre, wherein the DNA is retained by ionic bonds and enters and exits. When viewed under an electron microscope, the nucleosomes in the chromatin appear as beads-on-strings.

A proteinaceous superhelix with a left-handed structure and a pitch of approximately 28 A is indicated by a complex outer surface resembling a flat disk or a wedge in planar projection.

The core histones are 4 proteins referred to as H2A, H2B, H3 and H4 seen in equal parts in cells. All these four histone amino acid sequences comprise 20%-24% lysine and arginine. Protein size ranges between 11400 and 15400 Daltons, causing them to be comparatively smaller and still highly positively charged proteins. This high content of the charged (positive) amino acids enables them to be associated closely with the DNA (negatively charged).

The histone fold domains lead to the formation of heterodimers, which occurs when the core histones interact with the quasi-symmetric heterodimer. The symmetry enables the heterodimer to be placed on itself by a 180-degree rotation around the axis of symmetry. As a result, both ends of the histones involved in the binding of the DNA of the H3-H4 (crescent shape) are equivalent, yet still organizing the different DNA stretches. Similarly, the H2A-H2B dimer also folds. In the first step of the formation of the nucleosome, the H32-H42 tetramer is surrounded by the DNA. Subsequently, both the H2A-H2B dimers are linked to the DNA- H32-H42 complex, forming the nucleosome.

The 4 core domains each contain histone-fold domains as well as unstructured, flexible extensions known as histone tails. These tails can be subject to a variety of modifications, such as lysine and arginine residues, acetylation, methylation of serine and phosphorylation.

Histone Octamer Function

Histones are a group of proteins associated with DNA in the nucleus and aid in condensing them into chromatin. This nuclear DNA is highly condensed and surrounds the histones in order to be able to fit into the nucleus and participate in chromosomal formation.

Each histone octamer consists of two copies of histone proteins: H2A, H2B, H3, and H4. This series of nucleosomes is then enclosed in a spiral-shaped structure known as a solenoid, which is held together by the H1 histone proteins that are linked to each nucleosome, thus maintaining the structure of the chromosome.

The histone octamers not only compact DNA, but also play a role in the transcription of the DNA they surround. Interactions occur between the octamers and the DNA via both the N-terminal tails and core histone folds, creating physical and chemical bonds with the minor groove of the DNA.

Besides compacting the DNA, these octamers are also involved in the transcription of the DNA that they surround.

Nucleosomes & Histone Octamer

Nucleosomes, the basic units of chromatin, are dynamic and can be remodelled using ATP-dependent remodelling factors in order to package and regulate the expression of genomes in eukaryotes.

Nucleosomes are composed of a DNA segment of approximately 146 base pairs, tightly wrapped around an octamer of histone proteins, resembling a thread wrapped around a spool. This arrangement is further surrounded by a superhelical arrangement.

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##Frequently Asked Questions on Histone Octamer

Histones are generally positively charged.

Histones, due to their positive charge, facilitate tighter bonds with the negatively charged molecules of DNA.

A histone octamer is a structure formed by the assembly of eight histone proteins in a ring-like structure, which is a key component of the tertiary structure of DNA.

Histone octamers are essential for DNA packaging, as they form nucleosomes when the negatively charged DNA wraps around them. The DNA is held together by ionic bonds, and the linker histone H1 is associated with each nucleosome, allowing the DNA to enter and leave. This results in a series of nucleosomes forming a 10 nm fibre, which appears as beads-on-string structures under an electron microscope when observed in chromatin.