Repeated sequences in human DNA

Human DNA has been fractionated in Ag+Cs2SO4 and Hg2+Cs2SO4 preparative density gradients, and the fractions obtained have been centrifuged in neutral CsCl after extensive dialysis to eliminate Hg2+ and Ag2+. By centrifugation in Ag+Cs2SO4 a new satellite, called satellite DNA II, has been isolated from human DNA. It has a density of 1.693 g/ml. in neutral CsCl, accounts for 2% of the total approximately, renatures rapidly and separates into complementary strands having different densities in alkaline CsCl. In Hg2+Cs2SO4 gradients human DNA appears to be composed of two classes of molecules. The first, which accounts for approximately 80% of the total, is highly heterogeneous in base composition, its density in CsCl ranging from 1.690 to 1.720 g/ml., and is distributed in Hg2+Cs2SO4 so that the A·T-rich fractions are on the heavy side and the G·C-rich fractions on the light side, as expected on the basis of the preferential binding of Hg2+ to A·T pairs. The second class, which accounts for approximately 15% of the total, is more homogeneous, has a density of 1.696 g/ml., and is located on the light side of the DNA band in the Hg2+Cs2SO4 gradient. This suggests that the amount of Hg2+ bound to this A·T-rich DNA is abnormally low. This second class of DNA has been isolated by preparative CsCl centrifugation from a pool of the light fractions obtained from DNA-Hg2+Cs2SO4 centrifugation. It tends to renature after heat-denaturation, as shown by the shift of its density towards the native value in neutral CsCl.