EFFECT OF RADIATIONS ON PHAGE PARTICLES 69 



quanta without being killed, and hence that ultraviolet light can 

 produce more than one kind of lesion. The growth delay in- 

 creases with increasing dosage of ultraviolet light and is not 

 hereditary, in the sense that progeny of affected phage particles 

 reproduce themselves with normal latent period. The magni- 

 tude of the effect varies from one phage type to another, the 

 growth delay for a given dose of ultraviolet being much greater 

 with phages Tl and T7 than with T2. Setlow, Robbins, and 

 Pollard (1955) determined the ultraviolet action spectrum of the 

 growth delaying effect in Tl and found that wavelengths near 

 2,600 A, the absorption peak of nucleic acids, have the maximum 

 effect. This suggests that extension of latent period, like in- 

 activation, results from the absorption of radiation by nucleic 

 acid. 



c. Host Killing 



Luria and Delbriick (1942) discovered that ultraviolet in- 

 activated T2 phage particles, though no longer able to reproduce 

 themselves, are still capable of killing phage-susceptible bacterial 

 cells. By determining the proportion of surviving bacteria after 

 infection with various amounts of ultraviolet-inactivated phage 

 particles, these authors demonstrated that the adsorption of a 

 single irradiated phage particle suffices to kill the host cell. 

 A dose of ultraviolet light that reduces the number of plaque- 

 forming particles to lO"'' of their initial number leaves Va of the 

 original phages still able to kill susceptible bacteria. The host- 

 killing property of phage T2, therefore, is enormously more 

 resistant to ultraviolet light than the viability of the phage. 

 Heat, in contrast to ultraviolet light, destroys both of these 

 activities at the same rate. Microscopic examination of bac- 

 teria infected with ultraviolet inactivated T2 particles shows that 

 such cells do not multiply and do not lyse, Cohen and Arbogast 

 (1950c) demonstrated that infection of bacteria with heavily 

 irradiated T2 phages arrests synthesis of both ribonucleic acid 

 and deoxyribonucleic acid. 



Cytological methods were employed by Luria and Human 



