IONIZING RADIATION AND VIRUSES 97 



area and volume values require either a highly multiple genetic 

 structure for T-1 or else a long, thin genetic structure. The 

 best guess at present is a long cylinder of radius 26 A and length 

 1,900 A. 



For the two plant viruses so far studied, the "genetic" 

 part, which is essential for survival, seems to be very nearly 

 the whole virus, at least the unhydrated part of the virus. 



Returning to T-1, we can make some analysis of the increase 

 in latent period. Adams and Pollard's data indicates that X-ray 

 bombardment corresponding to 25 primary ionizations per 

 phage particle will just double the latent period. Now if we 

 suppose that there is some critical concentration of a virus 

 precursor which must be reached very rapidly, we can suppose 

 that the full burden of the manufacture of this special precursor 

 falls on the virus itself. Perhaps it is this which converts the 

 basterial metabolism so rapidly to virus synthesis. The 25 pri- 

 mary ionizations can be thought of as inactivating 25 molecules 

 responsible for this. Since the latent period is doubled by this 

 bombardment, the rate of precursor synthesis is halved, so 

 that 25 molecules are left, making an initial total of 50. 



Now, 15 deuterons produce the same effect. These deuterons 

 each produce 15 primary ionizations in the virus, or a total 

 of 225 primary ionizations. Thus the dense tracks of deuterons 

 make an inefficient use of the primary ionizations, which indi- 

 cates that each of the sensitive molecules is large — large enough 

 to include several primary ionizations. If these hypothetical 

 molecules are considered to be spherical, their radius must 

 exceed 50 A. 



Actually, if we speculate that the whole virus volume is 

 divided into 50 units, and consider these as spherical, the radius 

 of each is 75 A. This would make the deuteron and X-ray 

 data roughly fit. The molecular weight of each unit would be 

 roughly 1,000,000. 



These units are possibly nucleoprotein units of some kind 

 since the total nucleic acid in the virus far exceeds the assumed 

 purely genetic part. This type of estimate is a mixture of 

 speculation and reality. The fact that 25 primary ionizations 



