Abstract
DNA from D. pneumoniae when heated to temperatures where all the molecules are denatured still retains transforming activity. The activity is increased by slowly cooling the denatured DNA as well as by increasing the ionic strength and concentration of DNA during the cooling period. The reunion of the strands is specific and has been shown to occur between populations of denatured molecules of mutant strains of D. pneumoniae. The reunion does not occur between the DNA strands of pneumococcus and that from Streptococcus D or from organisms with widely varying base ratios. Hybrid molecules in which one strand contains the mutant marker and the other strand derives from wild-type DNA are active in carrying out transformations with respect to the mutant property.