The replication of DNA III. Changes in the number of strands in E. coli DNA during its replication cycle.

DNA has been isolated from synchronized cultures of E. coli 15(T)- at various times. At first the DNA was four-stranded (and indistinguishable in all respects from log phase E. coli DNA), but at the start of DNA synthesis the DNA was found to have halved its molecular weight and to have become two-stranded. This sample had all the properties of undenatured, double-helical DNA, and behaved in all respects like DNA from non-proliferating sources. The replication cycle of the DNA molecule has thus been shown to consist of an alternation between the four- and two-stranded forms, the latter being the conserved unit. The evidence provided by the three papers of this series with respect to DNA and chromosomal structure and replication is discussed and summarized.