Age-related changes in a spontaneously reassociating fraction of mouse DNA

Rapidly reassociating fractions have been isolated from mouse brain and liver nuclear DNA by hydroxyapatite fractionation of sonicated and denatured preparations incubated at a Cot of 1. When prepared by thermal denaturation, a subfraction of this DNA, representing approximately 0.6% of liver and brain DNA, has been shown to spontaneously reassociate at Cot 10(-5). This fraction increases 3-fold in DNA from old animals. When prepared by alkaline denaturation or treated with pronase, no age-related increase is observed, suggesting the presence of an alkali-labile stabilizing protein component in DNA from old cells. A portion of this fraction is observed by electron microscopy under denaturing conditions to consists of looped hairpin structures in DNA from old, but not from yound or mature animals. These structures are not seen after pronase treatment.