REPRODUCTION OF VIRUSES: A COMPARATIVE SURVEY 555 



penetratioD of some prophages into a susceptible protoplasm upon mating of 

 a lysogeiiic bacterium with a nonlysogenic partner. It seems safe to assume 

 that infection of a sensitive cell can be initiated by entry of the phage DNA 

 in any one of its possible states. It is almost superfluous to point out that 

 the possibility of infection of bacterial protoplasts with phage DNA promises 

 new insight into the relation between structure and function of viral nucleic 

 acid. Some protein component of the phage appears to play an essential 

 role in the infection of protoplasts (Spizizen, 1957). 



C. Kinetics of Replication of Vegetative Phage 



If phage specificity throughout its reproductive cycles is embodied in 

 DNA elements, the question arises of the kinetics of DNA rephcation in the 

 course of vegetative multiplication of virus. By what mechanism does 

 multiplication take place? Does it consist of repeated copyings of a single 

 template, used over and over? Or does it involve a series of reduplications, 

 in which the newly produced individuals serve in turn as sources for replica- 

 tion? In other words, is multiplication linear or geometric? The second alter- 

 native is verified by genetic observations on spontaneous phage mutations 

 (Luria, 1951). These mutations occur only during multiplication; the resulting 

 mutant phage particles are found among normal particles m the phage yield 

 from single bacteria. The clonal distribution of the mutants in individual cells 

 fits a distribution predicted by the hypothesis of geometric multiplication 

 and incompatible with the hypothesis of a linear kinetics. 



Current ideas on the structure and rephcation of DNA are compatible 

 with its role as a geometrically replicated genetic material (Watson and Crick, 

 1953). A DNA molecule consists of two complementary polynucleotide chains. 

 Its replication must involve the formation of two new complementary chains. 

 The four chains will then yield two indistinguishable DNA molecules, pre- 

 sumably equal to each other in reproductive capacity. 



Phage rephcation must also allow an exact homologous pairing between 

 viral elements in order to account for the observed phenomena of genetic 

 recombination. Pairing and recombination can also be accounted for in terms 

 of mating during DNA rephcation (Delbriick and Stent, 1957; Levintlial and 

 Thomas, 1957a), although more complex schemes invoking mating between 

 non-DNA intermediates may ultimately prove preferable (Stent, 1958). The 

 possibihty of interactions similar to recombination between genetic elements 

 of the phage and of the host is also suggested by a number of genetic observa- 

 tions, as discussed in Vol. II, chaps. VII and VIII of this work. 



D. Functions of the Phage Genome 



Viewed as functional DNA, the vegetative form and the prophage form 

 of a bacterial virus are basically similar to fragments or portions of cellular 



