Fractionation of DNA II. Partial strand separation as a basis of fractionation by countercurrent distribution

Fractionation of native DNA from mammalian and bacterial sources by countercurrent distribution has been found to depend on the degree of partial separation of the two strands. This is indicated by thermal denaturation of countercurrent distribution fractions of DNA and by countercurrent distribution of DNA reacted with [¹⁴C]formaldehyde. Countercurrent distribution of DNA double-labelled with [¹⁴C]thymidine and [³H]deoxycytidine, and measurements of T_m and buoyant density in CsCl of countercurrent distribution fractions of DNA indicate that base composition does not play an important part in the separation. Countercurrent distribution fractions of DNA did not show significant differences in s values. Countercurrent distribution in the solvent system used may exaggerate preexistent partial denaturation but it is considered that the patterns obtained do reflect the amount of denaturation which is present in the DNA in the cell.