BACTERIOPHAGE 1 1 



middle of the latent period; afterwards, their number increases at an 

 approximately linear rate, as shown by Doermann's breakage experi- 

 ments (8). Using mixed infection with different mutants, as in Her- 

 shey and Rotman's experiments (17), Doermann has recently proved 

 that the very first crop of active particles to appear inside infected bac- 

 teria already comprehends the same variety of parental and recom- 

 binant types that are present in the final yield, and in the same 

 proportions (9). This indicates that the interactions leading to a re- 

 combination must take place in the early stage of infection, at a time 

 when no fully active particle can be recovered. As in a well-ordered 

 kitchen, the basic cooking appears to have been done before the first 

 course is served. It seems indeed a reasonable working hypothesis to 

 assume that the active phage particles, which appear at a linear rate 

 in the late phases of intracellular growth, are the end products of re- 

 production and may, therefore, play no role in further phage pro- 

 duction. 



The complexity of the interactions that occur in the early period 

 of phage infection is evidenced by several observations. Multiparental, 

 rather than only biparental reproduction, must be assumed if we want 

 to account quantitatively for the multiplicity reactivation of ultraviolet 

 inactivated phage by the genetic recombination hypothesis (25). Tri- 

 parental reproduction was directly proved by Hershey and Rotman 

 using mixed infection with three different T2 mutants (16). For these 

 and other reasons, all of which suggest repeated and independent 

 contributions of each infecting particle to the formation of several off- 

 spring, I have proposed the hypothesis (24) of an independent replica- 

 tion of the genetic units composing the phage, followed by their 

 reorganization into complete, mature phage particles. It should be 

 clearly remembered that no independent evidence for this mechanism of 

 independent reproduction of units has as yet been obtained. 



This seems to be as far as we can go at present in analyzing phage 

 production from evidence supplied by the end products. The bio- 

 chemist has recently thrown some interesting light on phage repro- 

 duction, approaching it from the direction of the non-specific building 

 blocks. The main results (see 3), obtained by determination of total 

 protein and nucleic acids in infected bacteria and by isotope techniques, 

 indicate that the material of the phage particles — which consist only 

 or almost only of protein and desoxyribonucleic acid (DNA) — derives 

 in the greatest part from compounds assimilated from the medium 

 after infection. The rate of assimilation of these new materials is 

 similar to the rate of synthesis of bacterial protoplasm in non-infected 

 cells immediately before infection. This suggests that the pre-existing 



