DNA-Binding Proteins and Their Interactions with Structure-Building Ions

The organization of DNA into a chromosome structure is implemented by its interactions with specific classes of protein molecules. In eukaryotes five different proteins, the histones, are found associated with chromosome DNA. The histones can be subdivided into two sets, on the basis of their roles. The first set comprises the so-called core histones, H2A, H2B, H3 and H4, organized in an octamer around which about 200 base pairs of DNA are wound in two loops constituting the fundamental chromatin particle: the nucleosome. The second set of eukaryotic histone molecules comprises those of the H1 type, which combine with DNA independently of the "core" histones, stabilizing the nucleosome and participating in the formation of higher-order chromatin structures. There are a variety of H1 histones in different organisms, cell types and in each cell.