Three methods of fractionating DNA are described, in which DNA fragments are separated on the basis (1) of their base composition, (2) of the ability of denatured DNA to renature after incubation, and (3) of the presence of intrastrand secondary structure in denatured DNA. The ability of some of these fractions to form specific duplexes with whole denatured DNA isolated from the same and from a different mammal has been investigated. The results are presented to demonstrate how the fractionation procedures described may provide new information about the organization of the mammalian genome.
