III. BACTERIOPHAGE DNA AND BACTERIAL DNA 141 



degree, however, for all experiments indicate that the transfer is dis- 

 persive, and that the difference is in the degree of dispersion. It is not 

 unlikely that the degree of dispersion depends on physiology of the 

 infected bacteria. At this point there has been only limited effort to 

 reconcile these apparent differences. The transfer studies using P''- 

 autoradiographs have recently been reinvestigated by Kahn (1961), 

 who finds in general smaller and more heterogeneous parental pieces in 

 the transferred phage than was found by Levinthal. 



4. Transjer of Lambda DNA 



At the present time there have been no published experiments which 

 have attempted to trace the distribution of parental DNA into progeny 

 lambda DNA molecules, although density gradient studies have been 

 done on whole phage. Meselson and Weigle (1961) have shown that the 

 progeny lambda phage collected from a single cycle of infection by an 

 N^^-C^^-labeled parent produce some infectious phage which have a 

 density corresponding to phage particles made of totally new protein 

 and totally heavy, parental, DNA ("conserved" phages), and another 

 peak having the density expected for phage having half parental 

 and half newly made DNA enclosed in newly made protein ("semi- 

 conserved" phages). Phages with recombinant genotype were shown to 

 contain parental DNA because they were found at certain positions 

 between those expected for fully conserved and totally light or newly 

 synthesized phage. Experiments of a similar nature have been performed 

 by Kellenberger, Zichichi, and Weigle (1961a) using phage which have 

 natural density difference as a result of a difference in DNA content 

 (Kellenberger, Zichichi, and Weigle, 1961b). Again phages of recom- 

 binant type w^ould be shown to contain preferred quantities of parental 

 DNA. These experiments have been discussed more fully in Chapter II. 

 It is important to note here, however, that the transfer in lambda is 

 quantitatively different from that found in T2 or T4 phage. The 

 parental DNA is not dispersed into pieces which are small (compared 

 to the total DNA content of the lambda phage) and dispersed into many 

 progeny phage. 



II. Bacterial DNA 



At the present time we have much less precise knowledge of the 

 organization of bacterial DNA's than we do of bacteriophage DNA's. 

 Again what information is available has come from the interpretation 

 of cytological studies, growth and transfer experiments, the study of 

 purified DNA, and genetic experiments. 



