On the Transfer of Information from Old to New Chains of DNA Duplexes in Phage Lambda: Destruction of Heterozygotes

The Watson-Crick model for DNA duplex duplication proposes that the two parental chains separate and that each directs the synthesis of a complementary chain with which it is found associated after the duplication act. Previous experiments have left unchallenged alternative models which propose that in any single act of duplication only one of the two parental chains provides information for the synthesis of both new chains. The models are operationally distinguishable since the former demands that heteroduplexes are destroyed by duplication while the latter anticipates their survival. We have shown for phage lambda that duplication destroys heterozygotes as predicted by the Watson-Crick model. A stock of lambda containing a high frequency of heterozygotes at the cI locus was prepared by conducting a cross under conditions of depressed DNA synthesis. Particles in this lysate were permitted to duplicate a few times by adsorbing them to a lambda lysogen in a15N13C medium along with a heteroimmune lambda strain. Emerging lambda particles were separated according to density. The population of particles carrying DNA of parental density retained the initial high heterozygote frequency. Among particles which had duplicated, 80 per cent or more of the heterozygotes had disappeared.