A study of foldback DNA.

Nuclear DNA from eucaryotes contains a significant fraction which forms duplexes very rapidly and also independently of the DNA concentration. This fraction can be isolated by adsorption to hydroxylapatite and has been called foldback DNA (Britten and Smith, 1970). Here we extend previous studies to show that the foldback fraction is due to the existence of a finite number of foldback foci in each genome equivalent of DNA, approximately 10(5) in the case of Xenopus laevis. More significantly, we have isolated the foldback fraction in quantity from DNA of such a size (in one case broken randomly and in another digested with a restriction endonuclease) that only about 10% of the total DNA has foldback properties. If the foldback foci were located in precisely the same positions in all sets of the Xenopus laevis genome, the prediction would be that these foldback fractions would contain sequences representing 20% (random shear) and 10% (restriction endonuclease) of the total genome. In contrast, our results show that in both cases the foldback fraction contains the entire Xenopus laevis DNA sequence. One possible explanation of these observations is that as in procaryotes, eucaryotic DNA is randomly cross-linked. We show that cross-linkage of Xenopus laevis DNA is not sufficient to explain our observations. In consequence, we have adopted the hypothesis that the formation of foldback DNA is mainly an intrastrand phenomenon, but nevertheless occurs at different sites in different sets of the Xenopus laevis genome.