UADIATION AND VIRUSES 355 



times greater than that of bacteria with two particles. Dulbecco's work 

 on photoreactivation of phage (1951, unpiibhshed results) suggests that 

 some phages (the T2, T4, TG group, in particular) may receive two types 

 of ultraviolet damage, one photoreactivable by a one-quantum process, 

 the other by a multiple-hit process. Multiplicity reactivation can over- 

 come the effects of both types of damage. It may involve some kind of 



140 



280 420 560 



ULTRAVIOLET OOSE.Sec 



Fig. 9-1. Survival of phage T2r and its multiplicity reactivation after ultraviolet 

 irradiation. Exposure was at 80 cm distance from a 15-watt germicidal lamp 

 (General P^lectric Company). Broken line, free phage survival; solid line, reactiva- 

 tion frequency (fraction of mvdtiple-infected bacteria that liberate active phage). 

 The figures given for each solid-line curve indicate the average multiplicity of infec- 

 tion in the whole population. (Modified frotn Dulbecco, 1952.) 



very efficient cooperation at the physiological level, together with a 

 mechanism of genetic recombination of a more orthodox nature than the 

 one postulated by the "gene-pool" theory. 



For phages inactivated by X rays, multiplicity reactivation is very 

 slight, with a frecjuency much lower than with ultraviolet-irradiated 

 phage (Watson, 1950). It has been suggested that it may occur only for 

 that fraction of particles that are inactivated by acts of X-ray adsorption 

 which resemble ultraviolet (juanta in the extent of damage they produce, 

 and possibly in the amount of energy released. Interestingly enough, an 



