BACTERIOPHAGE GENETICS 303 



As a model of the genetic phenomena, we consider a pool of vegetative 

 particles which can multiply and mate but which are not themselves infec- 

 tious (Visconti and Delbriick, 1953). The process by which these vegetative 

 phages are converted into complete phage particles will be called maturation, 

 and particles will be thought of as being extracted from the mating pool for 

 maturation. The mature phage particles become converted to the vegetative 

 particles at the time of injection into a new bacterial host, and the matura- 

 tion is irreversible within any cell. After the infection process starts, the 

 vegetative phage contributed by the parental particles multiplies and the 

 pool size of vegetative particles increases until it becomes stabilized by ex- 

 traction for maturation. It reaches a stable level at which the rate of multi- 

 plication of particles in the pool is just equal to the rate at which particles 

 are extracted from the pool. Mating takes place in this pool at a rate which 

 for the phage T2 is approximately as high as that of the multiplication 

 (Le\'inthal and Visconti, 1953), but in the phages Tl and A is very much 

 lower. 



B. Distribution of Mutants 



If a mutation should occur in this pool of vegetative particles, the total 

 number of mutant phage it would contribute to the final burst would depend 

 on when during the latent period it occurred. Since the pool is constant in 

 size, every particle, once it is formed, must have a probability of one-half of 

 being extracted from the pool and an equal probability of undergoing further 

 duphcation. If it is duplicated, then each of the two daughters has a prob- 

 ability of one-half of being extracted and one-half of undergoing further 

 growth to produce additional particles. Thus, except for the effect of muta- 

 tions which occur just prior to lysis and before the pool reaches constant size, 

 50 % of them would lead to only a single mutant phage particle in the final 

 mature progeny, and approximately one-eighth of the mutational events 

 would yield the two daughter mutants in the burst. The distribution of 

 mutants per mutational event can be calculated (Levinthal, 1957) by con- 

 sidering successively the probability of extracting daughters, granddaughters, 

 etc., and the results can be represented to a good approximation as follows: 



^(^) = ^ (1) 



^(x) is the probabihty of finding a clone of size x in the progeny of a single 

 burst. Luria (1951) measured this distribution for the phage T2, and from the 

 observed results he was able to conclude that a mutant which arose in a cell 

 was itself able to multiply and produce a clone of mutants. The distribution 

 he obtained was not significantly different from that predicted by equation 

 (1) except that it had more very large clones than the calculated distribution. 



