Histones are proteins that coil DNA tightly in eukaryotic cells and some microorganisms. They play a role in gene expression and maintaining DNA structure. The most common histone proteins are H1/H5, H2A, H2B, H3, and H4. Histones form an octet structure with DNA to create nucleosomes, which form chromatin fibers and chromosomes. Histones are relatively conserved across species and are necessary for cell survival.
Histones are structures in eukaryotic cells and some unicellular microorganisms of the phylum Euryarchaeota that serve as coils around which the cell’s deoxyribonucleic acid (DNA) coils very tightly. Without the conservation of space afforded by histones, cells could not contain their own DNA. Histones also play important roles in gene expression by allowing or hindering the access of transcription-active molecules to DNA genes. A third task is to maintain the structural integrity of the DNA and the much larger chromosome.
The substances that make up histones are proteins that differ little from species to species. The most common proteins are called H1/H5, H2A, H2B, H3 and H4. DNA is tightly bound to histones by the attraction between the side groups of histone proteins and DNA. This attractive force is modified by the addition of acetyl or methyl groups to some lysine or arginine amino acids near the end of the H3 and H4 proteins. The shrinking or loosening of the DNA strand causes genes to be accessible or inaccessible, known as gene activation or deactivation.
In most cells, regardless of source, eight histone proteins, consisting of two each of H2A, H2B, H3 and H4, form an octet structure. About 146 base pairs of DNA wrap around the octet structure nearly twice to form a “nucleosome.” A short loop of DNA, stabilized by the H1 protein or its H5 analogue, leads to the next nucleosome, forming a structure that is often characterized as “beads on a strand”. Nucleosomes and their connecting DNA sections form tight coils, with six nucleosomes per turn, to form so-called chromatin fibers. The fibers join together to form a chromosome.
Histone proteins H2A, H2B, H3 and H4 have a relatively low molecular weight, consisting of 120 to 135 amino acids per protein molecule. Histone H1/H5 are much longer and give a structural framework to nucleosomes, much like a steel rod connecting a series of discs. In human cells, if all of the DNA were uncoiled and laid end to end, the strand would be about 70 inches (1.8 m) long but only about 0.0000007 inches (180 nanometers) thick. By twisting and turning the substructures, the 23 pairs of chromosomes function in a nucleus that is itself less than 0.0004 inches (10 micrometers) in diameter. Histones make this fold
possible by controlling the molecular environment.
Histones were originally thought to have only the types mentioned above. Research, however, has indicated far more diversity than previously accepted. The basic molecules are still relatively the same even between divergent organisms such as yeast and mammals. This trait is called evolutionary conservation. It indicates that even minimal variations of these molecules give rise to cells that could not thrive or would reproduce and cause damage and evolutionary penalties to the organism.
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