186 THE PHYSICS OF VIRUSES 



very commonly. In common genetic terms, they are on the same 

 linkage group and close together. 



To explain their results, Hershey and Rotman (1949) use three 

 linkage groups and 17 total genes. 



In all of the above, only bi])arental recombination has been 

 considered. If, now, infection is carried out with three phages 

 differing in three genetic loci, a much more complicated pattern 

 results. Delbriick (1951) has indicated how this may be analyzed, 

 and has applied his treatment to some of the simpler experi- 

 mental trials. The remarkable result emerges that the theoretical 

 analysis stands up in the face of experimental findings, but that 

 four rounds of mating between genetically com])leted viruses 

 must take place in the time of a single burst. This means four 

 rounds in a few minutes, since it certainly takes a major part of 

 the latent period to assemble genetically finished phage. This is 

 very rapid and is surprising, but at present has to be accepted as 

 in the nature of virus genetics. 



The two major conclusions to be drawn from virus genetics 

 are, therefore, that there are many genes ])er virus, possibly 20 

 to 100 in number, and that viruses which are complete as regards 

 their genes can "mate" inside the bacterium and do so very 

 rapidly. A secondary conclusion is that some of the genes are 

 linked, or at any event combine with one another with low 

 probability. 



Before trying to see what this entails in terms of physical 

 structure, we consider a second important and recent line of work 

 on phage multiplication. 



Bacterial Virus Multiplication 



The simplest physical idea of virus multiplication is the tem- 

 plate idea. According to this, the entering virus, or a j^art of it, 

 is a template which, acting as a firm structure in the midst of the 

 metabolic turmoil of the bacterium, causes copies of itself to be 

 formed by what amounts to a printing press operation. Although 

 all features of the idea are not simple, the notion that short-range 

 physical forces compel the aggregation of like particles around 

 the template, and hence produce a replica, is not unattractive. 



