184 THE PHYSICS OF VIRUSES 



used by an astute theorist to predict undiscovered properties of 

 viruses. 



We have already seen that certain aspects of virus action, most 

 notably the first stage of attachment to the host cell, are very 

 readily described (though so far no exact theory has been devel- 

 oped) in terms of electrostatic attraction. There seems to be no 

 reason to believe that any enzymatic processes which follow are 

 in any way demanding of new principles, because enzyme action 

 in general seems to conform to basic physico-chemical principles. 

 However, neither of these can be more than rather rudimentary 

 aspects of virus behavior. At the time of writing, the most pene- 

 trating studies of viruses are concerned with the phenomenon 

 of virus recombination and the genetic studies which have en- 

 sued. Therefore, even though it takes us beyond the confines of 

 physical studies, we describe here some of the genetic findings, 

 because these must be part of an over-all physical description of 

 virus action when (and if) it is one day complete. 



Virus Recombination and Virus Genetics 



Viruses which are essentially similar to one another can, in 

 some instances, be distinguished by characteristic behavior. For 

 example, of two viruses, one can have a short latent period and a 

 large burst size and so can produce lysis at a rapid rate. The 

 plaques so formed are then distinguishable from another quite 

 similar virus which lyses more slowly and produces smaller 

 plaques. Or again, two closely related bacterial hosts may act 

 as indicator for one but not the other. 



In 1946, Delbriick and Bailey made the important observation 

 that if two phages of diflPerentiable type infected a host cell, then 

 the resulting burst contained new types of phage differing from 

 the originals, and which retained their behavior after many 

 future generations. This process of recombination has given rise, 

 largely owing to the work of Hershey, to a whole developed sub- 

 ject of bacteriophage genetics which is of the greatest interest. 



Such recombination has also been observed in influenza virvis 

 by Burnet and Lind (1951). They have shown that three strains: 



