[Planar model of nucleosome and chromatin structure of high orders].

A model of nucleosome is discussed, which consists of two nucleohistone strand folds, located at the same level, similarly directed and having a rhomboid form. The folds are symmetric to each other. Four histones (H3, H2a, H2b and H4) take part in the formation of each fold. Nucleosome begins with a DNA region, bound with H1 histone and terminates with free DNA. Total sequence of histones along DNA is H1-H3-H2a-H2b-H4-H4-H2b-H2a-H3. Polypeptide chains of neighboring histones are oppositely directed and are located at opposite DNA strands. The model explains regularities of chromatin splitting under combined effect of ds-nucleases and trypsin, and of ss-nucleases. It is also in a good agreement with other properties of chromatin. Nucleosomes join to each other "side-to-side" under coincidence of terminal elements with faces of the rhomboid nucleosome structures. The model permits to explain the formation of a highest order structure--a helix of six nucleosomes, forming a fibril of 250--300 A in diameter. The degree of DNA compactness in it reaches 80--100.