6 BACTERIOPHAGES 



mature temperate phage particle infects a susceptible bac- 

 terial cell it may be transformed either to the vegetative state 

 leading to destruction of the host cell, or to the prophage state in 

 which it enters into the hereditary symbiotic relationship with the 

 host cell known as lysogeny. This relationship has been termed 

 symbiotic rather than parasitic because it may be readily sur- 

 mised that the lysogenic condition is of great survival value to 

 both phage and host cell. 



The recognition that temperate phages could exist in three 

 distinct states served to unify the precursor and virus theories. 

 The endogenous precursor in lysogenic bacteria is simply the 

 phage-specific prophage state of temperate phages. This 

 synthesis of the opposing concepts was clearly foreseen by Burnet 

 and McKie (1929). The status of precursor theories as applied 

 to infection by exogenous phages is still unclarified, however. 



Recent research has demonstrated a marked similarity in 

 physical, chemical, and biological properties between bacterio- 

 phages and other viruses. The similarity has stimulated much 

 research in which bacteriophages have been used as models for 

 animal viruses, even in extensive screening programs for anti- 

 biotics against viruses. To a limited extent this procedure has 

 already justified itself, but it is doubtful whether the homology 

 can be pursued very far. It is possible that bacteriophages and 

 other viruses have evolved from quite distinct ancestors, as the 

 following paragraphs suggest. The facts that the known bac- 

 teriophages contain deoxyribonucleic acid whereas several typi- 

 cal plant and animal viruses contain ribonucleic acid may prove 

 more significant than the rather casual resemblances demon- 

 strated so far. 



4. Origin of Bacteriophages 



Theories of the origin of bacteriophages have been closely 

 associated with theories of their nature. Many diverse ideas 

 have been proposed, most of which fall into one of two cate- 

 gories, the "creation" hypothesis or the "degeneration" hypothe- 

 sis. The creation hypothesis considers that viruses are the direct 



