BACTEEIOPHAGE AS A MODEL OF HOST-VIRUS RELATIONSHIP 191 



When analyzing the various phases of the life cycle (Table I), it seems as 

 though the genetic material, the specific nucleic acid carrying the information, 

 is the only structure which the three phases have in common. The multiplica- 

 tion of the genetic material takes place during two phases: in the lysogenic 

 bacterium and during the vegetative phase. In a lysogenic bacterium, no 

 phage proteins have been so far detected and it would seem that phage 

 proteins are synthesized only during the vegetative phase. The vegetative 

 phase is the only pathogenic phase of the cycle and the phage particle the 

 only infectious phase. 



TABLE I 



Constitution and Peoperties of a Bacteriophage at the Various Phases of 



ITS Life Cycle 



A bacteriophage is obviously the sum of the various phases of its life cycle. 

 It would be arbitrary to decide that supremacy should be given to one phase 

 rather than to another: multiplication of the genetic material, synthesis of 

 proteins, and infection are parts of the life cycle. A bacteriophage is therefore 

 a strict parasite of bacteria possessing an organized infectious phase. It is 

 produced from its genetic material and it is multiplied as genetic material. 



III. The Origin of Bacteriophage 



Most of those who have discussed the problem of the origin of bacterio- 

 phage have considered the infectious particle. Let us instead consider briefly 

 the problem of the origin of the prophage. 



Two main theories have been proposed: (1) the prophage is the residue of 

 the degradation of a parasitic bacterium or of a more or less primitive organ- 

 ism; (2) the prophage was born by genie or chromosomal mutations of a 

 bacterium which thus became lysogenic. Instead of trying to discover which 

 theory is right, let us try to build a third theory which would suppress the 

 opposition. 



Bacteria are a very homogeneous and certainly monophyletic group, which 

 differ from all other protista by the organization of the mitochondrial 



