7.4: The Nucleosome
DNA in a human cell is almost 2m long and it is packed inside a tiny nucleus that is only a few microns in diameter. The level of compaction of DNA inside the nucleus is astonishing. It is organized into several sequentially higher levels of compaction to fit into such a tiny space. The most compact form of DNA is a chromosome that can be seen under a microscope in a dividing cell.
DNA is wound twice around a protein complex called histone core, that consist of 8 histone proteins. This complex of DNA and histone protein is called the nucleosome, the fundamental and functional unit of DNA compaction. Nucleosomes can further coil around themselves into higher order compaction.
When the DNA is extracted from cells in low salt conditions and examined under a microscope, it resembles the beads on a string. The string represents the free DNA called "linker DNA," connecting the bead-like nucleosomes. If the DNA is isolated in physiological salt conditions (0.15 M KCl), it assumes a fiber-like form with 30 nm diameter that is bound to H1, a nonhistone protein. The H1 protein tightly binds to both the nucleosome and does not allow the DNA to slip.
Histones are highly conserved proteins
The amino acid sequences of core histone proteins are highly conserved between distantly related species. For example, the amino acid sequence of H3 histone between calf thymus and pea plant has only four amino acid differences.
Nucleosomes complex is also bound by a small proportion of nonhistone proteins, which help in maintaining the compaction and organizing long chromatin loops. Nonhistone proteins are also involved in the regulation of DNA replication and RNA synthesis.