Heterogeneity of base composition of DNA extracted from human leukaemic leucocytes

The DNA extracted from human leukaemic leucocytes forms a band (density=1·699 g/cm3) asymmetric to the denser side in a CsCl gradient at equilibrium in the analytical ultracentrifuge. Such asymmetry is present to some extent also in every normal mammalian DNA. It is due to heterogeneity of DNA base composition, as shown by its persistence after treatments (renaturation, digestion with Mustelus canis liver deoxyribonuclease) upon which the asymmetry should disappear if it were due to denatured DNA molecules. By CsCl preparative centrifugation of leukaemic DNA, a DNA fraction displaying a density of 1·711 g/cm3 has been isolated from the denser side of the band. This fraction is formed by native double-stranded DNA, as shown by the density shift and the increase in optical density upon thermal denaturation. The extent of asymmetry of DNA band seems to be greater and more variable in leukaemic DNA (particularly in the case of chronic myeloid leukaemia) than in normal mammalian DNA. The interpretation of this finding is discussed.