xiv Introduction 



Luria examined the individual phage yield of many thousands of phage- 

 infected bacterial cells and scored the single bursts for the presence of progeny 

 viruses possessing a certain plaque-type mutant character. On the basis of 

 the observed clonal frequency distribution of these mutants, Luria was able 

 to infer that the replication of the hereditary material of the phage is geo- 

 metric, i.e. that it proceeds by a number of successive cycles of self-duplica- 

 tion, since other conceivable reproductive models, e.g. successive replications 

 of the initial parental element or chain replication of the last element produced, 

 would have led to mutant distributions quite diflFerent from that actually found. 

 In 1946, a most important discovery was made independently by Delbriick 

 and Bailey ( 44 ) and by Hershey ( 67 ) , who examined the genetic character of 

 the phage yield issuing from bacterial cells infected with two related parent 

 viruses differing from each other in two mutant factors. It was found that 

 among the progeny of such mixed infection there appear virus offspring carry- 

 ing one of the mutant factors of one and one of the mutant factors of the other 

 of the two parents, demonstrating that bacterial viruses can undergo genetic 

 recomhirmtion. The first detailed study of genetic recombination in phage 

 was undertaken by Hershey and Rotman (77), their paper being included in 

 our collection. This work showed that, on the basis of the frequency with 

 which recombinant progeny for various mutated characters appear in such 

 "crosses," it is possible to construct a genetic map of the phage on which the 

 mutant loci can be arranged in a linear order. Hershey and Rotman also 

 examined the frequency of complementary recombinant types in the yields of 

 individual mixed infected bacteria and found that the formation of comple- 

 mentary types does not seem to occur in a single event [Bresch (28) was able 

 to establish this conclusion even more convincingly in a later study]. This 

 fact led Hershey and Rotman to entertain the notion that recombination in 

 phage might not be the consequence of a reciprocal exchange of preformed 

 genetic structures, such as chromosomal recombination in higher forms, but 

 that it might be an act incidental to the replication of the genetic material 

 itself. This hypothesis, which came to be called "partial replicas'" (69), or 

 "copy choice" ( 97 ) , now forms one of the basic concepts in the understanding 

 of the molecular basis of self-duplication and genetic recombination. As more 

 data concerning the process of genetic exchange in phage accumulated, it 

 became evident that the theoretical analysis of a phage "cross" is a problem in 

 population genetics. It was seen that within each mixedly infected bacterial 

 cell, growth and recombination of the numerous vegetative phage replicas 

 proceed concurrently. In 1953, Visconti and Delbriick (150) developed, there- 

 fore, a theory which succeeded in explaining quantitatively the recombinant 

 frequency observed in different phage crosses under various conditions. This 

 theory assumes that replication and recombination of vegetative phages pro- 

 ceeds in an intrabacterial pool, in which phages repeatedly mate pairwise 

 and at random until lysis of the host cell, and from which pool the vegetative 

 phages are withdrawn irreversibly for maturation into infective progeny 



