Chromatin

Chromatin is that portion of the cell nucleus which contains all of the DNA of the nucleus in animal or plant cells. (A small amount of special DNA is also found in the mitochondria of the cell cytoplasm outside of the cell nucleus.)

DNA is never found as a naked molecule in animal or plant cell nuclei. DNA is always found in association with histone proteins (soluble in acid solutions), HMG proteins (soluble in neutral saline), residual proteins (soluble in concentrated urea solutions), phosphoproteins (soluble in basic solutions), RNA species (soluble in aqueous phenol solutions), and lipid species (soluble in chloroform-methanol solutions). By choosing the appropriate solution, it is possible to extract each of these classes of macromolecules away from the DNA and away from the other chromatin constituents.
Smaller molecules such as steroid and thyroid hormones and vitamins A and D are also found within the cell nucleus bound to DNA or to one of the other chromatin macromolecules. These small molecules are transported to the nucleus, where they play stimulatory roles for RNA and DNA synthesis on chromatin. Viral RNA species and DNA species can also play stimulatory roles on chromatin. Some RNA species are confined to the cell nucleus, where they are synthesized on chromatin and feed back on other segments of chromatin to influence RNA or DNA synthesis.

Foreign substances such as antibiotics (actinomycin D), dyes (acridine orange), enzymes (DNase I), and complex carbohydrates (phytohemagglutinin) also penetrate the cell nucleus to the chromatin, where they also may have stimulatory or inhibitory effects on RNA or DNA synthesis.

When cells divide, the chromatin is seen as distinct chromosomes, duplicating, with an equal partition of each set of chromosomes then traveling to each of the new daughter cells. When the new chromosomes reach the new cells, they begin to un-ravel into long thin extended 10 nm. (100 A.) microfibrils called euchromatin or condensed coiled masses called heterochromatin. The study of euchromatin and heterochromatin while within intact non-dividing cells, or after isolation from such cells, has revealed that RNA synthesis occurs only in euchromatin, and not in heterochromatin. Similarly, DNA synthesis is early in euchromatin and late in heterochromatin. Mechanisms for controlling RNA or DNA synthesis in chromatin can be studied by additions or deletions of macromolecules from either isolated euchromatin or isolated heterochromatin. Such studies reveal that histone proteins are largely inhibitory, while RNA and other proteins are largely stimulatory for RNA and DNA synthesis.

The Chromatin Network is dedicated to researchers and all others interested in the study of animal and plant chromatin as found in the cell nucleus. It is evident that by such study of chromatin, we will gain a deeper understanding of gene action and gene regulation, of hormone and vitamin action and regulation, of viral action and viral regulation, and of the general physiologic processes of embryogenesis, organ regeneration, the neoplastic process, and the immune process.