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Histones are alkaline proteins which play an important role in the packaging of DNA and the regulation of genes in eukaryotic organisms. Without histones, the unwound DNA in chromosomes would be very long, about 2 meters in length per cell. As a result, chromatin in the DNA is very tightly associated with these histone proteins, which package and order the DNA into structural units called nucleosomes. This supercoiled, condensed structure allows for the long DNA molecule to fit inside the nucleus. Selective winding and unwinding allows for portions of the DNA to be exposed. In this way, histones also play a role in regulating gene expression.

All Eukaryotes have five major classes of histones, each differing in molecular weight and amino acid composition. These classes are H1, H2A, H2B, H3, and H4. By using X-ray crystal structure, we are able to see that the bead on each nucleosome contains eight histone molecules, and two copies each of H2A, H2B, H3, and H4. This provides a repeating unit of about 200 base pair, of which 146 base pair are bound tightly around the eight-part histone core [1] and the remainder serves as linker DNA between nucleosome beads. The crystal structure deduced is shown below:

The eight histone proteins are shown in different colors, where blue represents H3, green represents H4, yellow represents H2A, and red represents H2B. The DNA double helix can be seen wound around the nucleosome core.

Besides the core histones, there also exists the linker histone, H1. This allows for multiple nucleosomes to be linked to each other. As a result, DNA is locked into place. Interestingly, histone H1 is very important in stabilizing chromatin higher-order structures, and 30-nanometer fibers form mo...

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