DNA Synthesis Associated with Recombination I. Recombination in a DNA-negative Host

Abstract

In 1961, the work of Meselson and Weigle and of Kellenberger, Zichichi, and Weigle demonstrated that a genetically recombinant chromosome in phage λ can be composed in large part of atoms derived directly from at least one of the chromosomes which interacted to form it. Thus, “copying choice” was ruled out as the sole mechanism of recombinant formation. As pointed out by Meselson (1964), the technique employed by Meselson and Weigle could in principle distinguish between two models involving breakage of parental chromosomes—a “break-reunion” model, and a “break-copy” model in which one end of the recombinant chromosome is composed of DNA from one parent and the other end is composed of new material bearing information from the other parent. However, in order to distinguish between the two models, it was essential to modify the Meselson-Weigle experiment. First, it was essential that both parents be density labeled, otherwise the origin of any light atoms in a recombinant molecule would be ambiguous. Second, the markers employed should be centrally located on the chromosome so that at least one parent could be expected to make a sizable material contribution to a recombinant. In his 1964 paper, Meselson demonstrated that recombination between genes J and cI (see Fig. 1) produced some recombinant particles containing chromosomes composed exclusively of atoms present in the infecting phage particles. These results were interpreted as demonstrating that some recombinants, at least, arise by “break-reunion” as opposed to “break-copy.” The force of that conclusion was weakened by two considerations.