Physico-chemical characteristics of DNA chromatin fractions from calf thymus.

Structural and functional differentiation of chro­ matin implies the search for the methods of isolation and characterization of DNA bound with chromatin fractions. Preliminary fractionation of chromatin fol­ lowed by the isolation of DNA from these fractions are the methods frequently used. The isolation procedure put forward by Strazhevskaya and Struchkov [1] makes it possible to isolate DNA fractions directly from the tissue homogenate. The prepara­ tions obtained in this way can be useful for the examination of functional and structural properties of chromatin DNAs. Strazhevskaya and Struchkov [1] isolated two DNA fractions from different mam­ malian tissues by their own method. They found that the fractions had different physico-chemical proper­ ties (specific viscosity, hyperchromic effect) and metabolic activity. The contents of these fractions in particular tissues differed and depended on the specificity of cell metabolism. On the basis of the results obtained they assumed that the isolated fractions correspond to euand heterochromatin DNAs. However, they characterized only some physico-chemical properties of these fractions. Litera­ ture data report the differences in the nucleotide composition of DNAs from euand heterochroma­ tin. It was shown that satellite DNAs are mostly located in heterochromatin [2-6]. Nucleic acids of both fractions differ in the respect of methylation degree [7], The differences of Tm values between euand heterochromatin DNAs were pointed out by