These are special proteins that are found in eukaryotes and archaea, but not in bacteria. In eukaryotes the histone protein provides structural support for the chromosome. The importance of the histone proteins to the nucleus has recently become clearer. In the human nucleus, for example, there is about 2 m of DNA in total. The nucleus is an awfully tiny volume to package all that DNA into. The solution is the nucleosome, one bead of which is made up of 8 histone molecules. In a single nucleosome, the DNA is tightly wrapped around the histone bead. The nucleosomes, in turn, coil tightly around each other. The result is chromatin, the obvious expression of a chromosome.

Chromatin, however, is not uniformly compacted across the entire genome. Some regions are more loosely packed than others. Genes which are expressed, are in the more loosely packed regions of the genome. Transitions from tightly to loosely packed DNA (closed to open chromatin) are important in cell differentiation.

Apparently the nucelosomes themselves are in almost constant tension, tightening, loosening and twisting. Flexible tails from the histone proteins are somewhat involved in these activities. Only when tails flex away from particular regions of DNA are the nucelosomes able to open.

Some organisms can manage without histones. Bacteria lack them, and eukaryotic dinoflagellates (important algae worldwide which include species causing bioluminescence and toxic red tides) also manage without histones. [Georgi K. Marinov et al. 2021. Transcription-dependent domain-scale three-dimensional genome organization in the dinoflagellate Breviolum minutum. Nature Genetics DOI: 10.1038/s41588-021-00848-5]

The dependence of most eukaryotes on histones in the nucleus is yet another major difference between bacteria and eukaryotes. Archaea possess histone proteins, but the order of the amino acids in the proteins is different from in the eukaryotes. Moreover, the archaeal chromosome is circular like that of bacteria, and the DNA for the archaea is loosely wound around histone protein structures. Thus, the archaeal nucleosomes are not as regularly organized or as stable as eukaryotic nucleosomes.

This is yet another design feature which shows the major differences between the three domains described by Carl Woese but to which many current biologists do not necessarily subscribe. Some specialists insist that there are only two domains with the eukaryotes coming from the archaea. Of course, other experts reject this idea. It all depends upon what characteristics one considers most important. Assigning such relationships is a very subjective enterprise.

It is apparent that the histones are extremely important in the correct functioning of the eukaryotic nucleus. They are part of the purposeful arrangement of parts which must be just right for the cell to operate. These finely tuned life processes of the eukaryotic cell could not have developed gradually.