33. NUCLEIC ACIDS OF THE BACTERIAL VIRUSES 221 



Similarly the decay of incorporated P 32 atoms in the DNA of a T-even 

 particle can prevent the appearance of individual genetic loci in progeny 

 particles from mixed infections with allelic nonradioactive particles. In 

 this case, the effect of the destructive event appears to be less discrete, 

 in that unlinked markers are not eliminated entirely independently. How- 

 ever, there is a definite correlation between linkage and joint elimination 

 of marker; the closer the linkage between two markers the greater is the 

 likelihood of dual elimination. 143 ' 144 



It has been proposed that inactivation by P 32 is a consequence of scission 

 of the DNA molecule. 52 On this hypothesis the recovery of genetic markers 

 from a P 32 damaged phage particle in a mixed infection may depend 

 upon incorporation of a fragment of its DNA into the DNA structure 

 of one of the replicating DNA molecules of the carrier phage. In this case 

 the loss of a particular genetic component will bear no simple relation- 

 ship to its DNA, and hence its P 32 content. 



The pattern of inactivation of the phage genetic markers by X-rays 

 administered to free phage appears to be very similar to that observed 

 with P 32 inactivation, and hence has not been useful in the estimation 

 of the size of such markers. 144 However, if the X-irradiation is adminis- 

 tered to the phage-bacterium complex after injection, multiplicity reac- 

 tivation and cross-reactivation can be demonstrated with efficiencies com- 

 parable to those observed after ultraviolet irradiation. This recent dis- 

 covery 90 • 144a may permit the use of X-ray inactivation to obtain estimates 

 of the size of genetic loci. 



b. Association of Genetic Loci with Parental DNA 



Bacteria simultaneously infected with two or more related phages (of 

 the T-even type) will support the growth of each type. Up to 30 phage 

 particles can, at least, contribute genetic markers to progeny. 145, 145a - 146 

 During such a mixed infection, genetic interactions occur and genetic re- 

 combinants appear. Whether such interactions involve actual recombina- 

 tions of parental DNA molecules (or of their progeny) or are produced 

 during replication by a "copy choice" method 147 " 149 whereby a new DNA 



143 G. S. Stent, Proc. Natl. Acad. Sci. U. S. 39, 1234 (1953). 



144 F. W. Stahl, Virology 2, 206 (1956). 



144 " J. J. Weigle and G. Bertani, Virology 2, 344 (1956). 



146 R. Dulhecco. Genetics 34, 126 (1949). 



14Ba R. S. Edgar and C. M. Steinberg, Virology 6, 115 (1958). 



146 However, it appears that no more than two phage particles can participate in 

 early stages of infection such as the complementary action of /II + A and /II + B 

 cistrons to permit mixed growth of T4 mutants on K12 (X). 42 



147 A. D. Hershey, Intern. Rev. Cytol. 1, 119 (1952). 



148 C. Levirithal, Genetics 39, 169 (1954). 



149 J. Lederberg, J. Cellular Comp. Physiol. 45 Suppl. 2, 75 (1955). 



