Chromatin

CHROMATIN, an intensely staining substance, composed of proteins, that is contained in the nuclei of animal and plant cells. Its main protein component is apparently the so-called nucleoproteins (see), although the question of the exact chemical composition of chromatin appears to be far from resolved at the present time. In some cases (nuclei of Gregarina), typical chromatin does not exhibit a reaction to nucleoproteins. Chromatin is certainly not a chemically individual substance, and its protein component apparently possesses a high degree of individual variation, which may determine the significance of chromatin as a carrier of hereditary properties. Depending on the ability to stain with basic and acidic dyes, two types of chromatin are distinguished: basichromatin and oxychromatin, which apparently are capable of transforming into one another depending on the functional state of the nucleus. The reaction to chromatin is staining by Feulgen. Along with the chemical diversity of chromatin, one must note very large differences in its morphology. Nuclei differ very significantly from one another in the quantity and distribution of chromatin within them. One can point out a series of transitions from very compact nuclei, completely filled with a lump of chromatin (lymphocytes, heads of spermatozoa of various animals), to the most common ordinary form, where chromatin is represented by a network of small, irregularly shaped grains and granules distributed in loops of the achromatic framework, and further to chromatin-poor nuclei (nerve cells, nuclei of mature egg cells). The distribution of chromatin in the nuclei of the larvae of the mosquito Chironomus is very peculiar: it is represented here by a spirally twisted thread, in which sections of basi- and oxychromatin alternate, which gives the chromatin of the nucleus the appearance of a variegated ribbon. The question of the distribution of chromatin in protozoa is also very interesting. Here, in particular, individual large accumulations of chromatin, so-called karyosomes, are frequently observed in the nuclei. The question of the presence of chromatin in bacteria and blue-green algae is somewhat controversial. Studies of recent years, conducted using fine microchemical reactions, have shown that in all these cases we are dealing with a diffuse distribution of chromatin in the form of the smallest grains. The question of the relationship between oxy- and basichromatin appears not entirely clear. Initially, the prevailing opinion was that the most active form of chromatin is basichromatin, the quantity of which increases significantly in the nucleus at the time of cell division; however, subsequent studies have shown that a similar process is observed during phenomena of cell degeneration—a circumstance that forces some researchers to attach decisive significance to oxychromatin, which, as the cell ages, transforms into an inactive form of basichromatin. The whole question is still far from being resolved. Similarly, there is no complete unity of views on the physiological significance of chromatin. Its participation in the processes of transmission of hereditary properties is well known, since the carriers of hereditary traits—chromosomes—as their most noticeable part have basichromatin. However, the number of voices in favor of the significance of the achromatic framework in chromosomes is increasing, while basichromatin itself is assigned a secondary and purely auxiliary role. On the other hand, it is possible that individual hereditary traits are linked with small grains of chromatin—chromomeres. The physiological significance of chromatin apparently also lies in its participation in metabolic processes (chiefly in enzymatic processes). The application of microchemical reactions shows that the period between two cell divisions (interkinesis) is characterized by an active state of the nucleus, during which chromatin exits into the cell plasma in a finely dispersed state and participates there in a number of processes (accumulation of yolk in the egg, secretory processes in glands, etc.). In all these cases, chromatin obviously plays the role of a catalyst stimulating the course of various chemical processes in the plasma. In a number of cases (especially in protozoa, e.g., in radiolarians), fairly large accumulations of chromatin, so-called chromioli, are described in the plasma, giving a typical reaction and, in the opinion of some, playing the role of permanent chemical centers in the plasma. The proposal to sharply distinguish genochromatin—the carrier of hereditary properties—and trophochromatin, managing metabolic processes, is hardly correct if one recalls that the quantity of chromatin in the nucleus is not constant and that, for example, the preparation of nuclei for fertilization (the transition of part of the chromatin to the plasma, reduction of chromatin) is accompanied by a significant reduction in its quantity; restoration occurs only gradually, apparently at the expense of the formation of new chromatin substances that enter the nucleus from the plasma. All the above forces one to recognize that both chemical and morphological unity of chromatin is absent, just as its continuous continuity in the transition from one cellular generation to another is absent. Chromatin retains its significance as a collective concept uniting a series of nuclear and partly cytoplasmic proteins characterized by their ability to stain sharply with certain (both basic and acidic) dyes and taking direct part in some of the most important biological processes (transmission of hereditary properties, enzymatic reactions, secretion).

S. Zalkind.