RECOMBINATION IN VIRUSES AND BACTERIA 



115 



pre-existing bacterial DNA is degraded and its breakdown products enter 

 a pool to which contributions are also made from the surrounding medium. 

 From this pool, constituents are withdrawn for the DNA synthesis which 

 soon begins again, now under the control of the virus. The virus-initiated 

 synthesis of new protein, which consists of many different species of mole- 

 cules, is a pre-requisite for this DNA synthesis, as is shown by experi- 

 ments in which the drug, chloramphenicol, is used to prevent the formation 

 of protein. When, as is sometimes the case, the virus DNA is unique in 

 containing hydroxy-methyl-cytosine instead of cytosine, the host is caused 

 to make this unaccustomed base. Even if the host is a mutant unable 

 to synthesize thymine, the infecting phage DNA has the information to 

 carry out this synthesis which is needed for its own multiplication. Thus 

 the viral DNA behaves throughout as a parasite; its genes replace those 

 of the bacterium, and the metabolic machinery of the host cell is re- 

 directed toward making phages. 



The time required for phage reproduction in a single bacterium is 

 short; after a period of often less than 30 minutes, the infected bacteria 

 burst and release some hundred newly formed phage particles. This 

 time is called the latent period. When the infected cell is opened arti- 

 ficially during the first half of this period, no phages are found, not even 

 the original infecting one. It has, of course, been dissociated, its pro- 

 tein remaining outside the cell after its DNA was injected. This DNA, 

 called vegetative pluige, multiplies during the early latent period and 



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20 



40 



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FIGURE 5.1. Some biochemical syntheses that are involved in the multiplication of 

 bacteriophages (after Cohen, in Luria, 1953, General Virology, p. 178, New York, 

 Wiley). 



