466 ROLLIN D. HOTCHKISS 



erties are in turn transmissible along with the new phage DNA, it is in- 

 ferred that the original DNA determinants in some way have recombined 

 to produce new determinants, just as they may do in bacterial transforma- 

 tions. It has been concluded from intricate analysis of this process that the 

 recombination must occur between nearly completed phage DNA's, since 

 recombinants usually are not replicated in the original host cell and do not 

 occur after maturation.^** 



A somewhat different kind of recombination in a mixedly infected cell 

 may result in the incorporation of one type of DNA within a protein mem- 

 brane corresponding to another type. This "illegitimate" phage bears for 

 one generation cycle only the host specificity of the membranes which it 

 has acquired, then reestablishes in the next infection cycle the normal 

 membrane and host specificity presumably corresponding to the DNA 

 which it had borne throughout this time.^*^'^^° 



Thus, although the detailed biochemical processes are by no means 

 understood, it is clear that the DNA of an infecting T phage commandeers 

 the nucleic acid and protein metabolism of the host cell much as though it 

 were a genetic entity dominant over the host genes. The specificity of the 

 new syntheses induced is predetermined by the infecting DNA and further 

 modification appears to occur only when, presumably, the DNA itself has 

 been modified in some manner. 



d. Origin and Fate of Phage DNA 



The various biological species have, of course, had to evolve pathways 

 of synthesis for the unique DNA's which they bear, and, in general, it 

 seems to be true that precursors at different levels of complexity may be 

 taken from the environment to build up this all-important material. Coli- 

 phages first tend to utilize host DNA but when this is used up, nucleosides 

 purine bases, and smaller components serve as sources of phage DNA.^*^' 

 176,181,191 'phg relative quantities used depend upon the requirement, i.e., 

 upon the DNA content of the phage produced,^*" and upon the number of 

 particles formed per cell at the time of assay. Hershey has shown that a 

 relatively constant amount of phosphorus and specific pyrimidine bases 



185 A. D. Hershey and R. Rotman, Genetics 34, 44 (1949). 



•" S. E. Luria and R. Dulbecco, Genetics 34, 93 (1949). 



188 N. Visconti and M. Delbriick, Genetics 38, 5 (1953). 



1*' A. D. Hershey, C. Roesel, M. Chase, and S. Forman, Carnegie Inst. Wash. Year- 

 book 50, 195 (1951). 



"0 A. Novick and L. Szilard, Science 113, 34 (1951). 



1" Of course, higher animals decompose and utilize fragments of ingested DNA in 

 their digestive systems, but Marshak and Walker''^ report that intravenously in- 

 jected chromatin itself is a relatively efficacious source of labeled phosphorus for 

 liver nuclei. 



192 A. Marshak and A. C. Walker, Am. J. Physiol. 143, 235 (1945). 



