33. NUCLEIC ACIDS OF THE BACTERIAL VIRUSES 



197 



10 



400 



20 30 



UV Dose (sec.) 



Fig. 1. Second cycle transfer of P 32 from irradiated parents as a function of ultra- 

 violet dose to the parents. The ordinate indicates the efficiency of transfer of P 32 

 from first to second cycle offspring, measured either in single infection (0.13 viable 

 phage per bacterium) or in mixed infection with live phage particles (5 live phages 

 per bacterium). [From A. D. Hershey and E. Burgi, Cold Spring Harbor Symposia 

 Quant. Biol. 21, 91 (1956).] 



dined from 1.0 to a limiting value of 0.45 with increasing ultraviolet dose 

 to the progenitor phage used in the first cycle. Thus, in this experiment 

 (Fig. 1) after heavy ultraviolet dosage, a maximum of 55% of the DXA 

 transferred by the initial irradiated phage to its progeny was transferred 

 into a small number of noninfective particles. 



This association of a large fraction of the transferred P 32 with noninfective 

 particles is interpreted to mean that the DNA of a T2 phage can be con- 

 sidered to consist of two functionally independent and physically separable 

 components roughly equal in mass. One component is effectively ultravio- 

 let sensitive and is not widely dispersed upon replication. Once irradiated 

 it bears lesions of such a character that any phage particle carrying this 

 DXA component is noninfective. The second component is effectively ultra- 

 violet resistant. Although it may acquire lesions upon irradiation, these are 

 either reparable or in any case are not lethal to a phage carrying such a 

 component. Studies with mixed infection with varying multiplicities of 

 irradiated and nonirradiated phage suggest that this second component is 

 separable from the ultraviolet sensitive component and is distributed es- 

 sentially at random among the infective and noninfective progeny of a 

 mixed infection. 



Similar experiments by Tomizawa 02 have shown that the DXA of a single 

 phage particle is not necessarily all synthesized in any brief time interval, 

 and specifically that the ultraviolet sensitive and resistant portions as de- 

 scribed by Hershey can be synthesized at considerably different times. 



