302 C. LEVINTHAL 



nature of the mating process. This difficulty of interpretation applies to aU 

 transfer experiments with phage, and various experiments are now under 

 way to study the transfer in single bursts and with additional genetic 

 markers in an attempt to clarify the point. 



VI. Mating Kinetics 



A. General Assumptions 



Although it is impossible to eliminate all other alternatives, one can en- 

 compass most of the data so far obtained in a self-consistent model of phage 

 growth and recombination. Probably any model which is proposed at this 

 time will have to be modified or extended as further experimental evidence 

 is obtained. It seems useful, however, to formulate in a reasonably precise 

 way the simplest model which at present appears consistent with the avail- 

 able data. To be sufficiently complete to allow detailed predictions to be 

 made, any model must specify two aspects of the processes which occur in 

 the infected bacterium. First, it must specify the nature of the elementary 

 mating act. One must make an assumption as to what goes into the mating 

 process in the genetic sense and what kinds of particles and how many arise 

 in it. Second, the model must specify the kinetic aspects of the mating pool, 

 how frequently mating occurs, whether the matings are random or synchron- 

 ized, and how the rate of extraction from the pool for maturation is related 

 to the growth rate. Unfortunately, almost any theory of the elementary 

 mating process can be made consistent with the experimental data if one 

 adjusts the assumptions as to the kinetics in the pool with sufficient freedom. 



Making the assumption that DNA is the sole carrier of genetic information 

 in the infected cell limits the type of model that one can construct to a con- 

 siderable extent, since an appreciable amount of data has been accumulated 

 about the behavior of the phage DNA during the process of growth in the 

 infected cell. We know from the work of Hershey (1953) that a pool of DNA 

 begins to be formed shortly after infection and reaches a size of approxi- 

 mately 30 phage equivalents per cell at about the time of the first appearance 

 of mature phage. After this time the pool size remains roughly constant until 

 lysis takes place. The DNA in this pool behaves as precursor material to the 

 DNA of the mature virus particles, and once it enters into the mature 

 particles it does not return to the pool. The DNA in the pool has a duplication 

 time of about 2 to 3 minutes, and in this time approximately 30 new mature 

 phage particles are formed. If the cells are prevented from iysing, the growth 

 and accumulation of mature phage particles inside the cells can continue for 

 as much as 90 to 100 minutes, at which time over a thousand newly formed 

 particles are, on the average, contained in each cell. 



