RADIATION AND VIRUSES 341 



microscopy or ultraceiitrifugal analysis and when calibrated ultrafilters 

 are unavailal)le. 



3. There is a progressively increasing discrepancy between particle 

 volume and target volume, proceeding from small to large viruses, par- 

 ticularly with sparsely ionizing radiation. Radiation sensitivity increases 

 more slowly than virus size.- This is where the weakness of the target 

 theory becomes evident. 



Lea (1946) favors the hypothesis that the target measures a true physi- 

 cal volume within which each ionization is effective. The smallest viruses 

 would be fully sensitive, whereas the particles of the larger viruses would 

 contain both a radiation-sensitive (genetic) portion and a nonsensitive 

 (nongenetic) portion. This analysis is carried further with the assump- 

 tion that the sensitive portion always consists of one or more spheres. If 

 the dependence on ionization density indicates that the effectiveness does 

 not decrease fast enough with increasing density, it is postulated that the 

 target consists of several spheres (a less wasteful arrangement for over- 

 lapping ionizations). Moreover, each sphere is compared to a gene, 

 whose lethal mutation results from a hit, and an estimate of the number of 

 genes per virus particle is derived ; for example, vaccinia virus would con- 

 tain about 100 genes, and a large phage would contain 10 or 20 (Lea and 

 Salaman, 1946). This type of analysis hardly seems justified. 



In the first place, there are neither radiochemical nor genetic reasons to 

 postulate a differentiation of biological materials into either indispensable 

 and fully sensitive to radiation or fully dispensable. For viruses, it seems 

 likely, in view of the complicated stages of interaction between virus and 

 host, that many portions of the virus may be altered by radiation in such a 

 way as to prevent reproduction. For example, surface groups may be 

 involved in adsorption onto the host cells ; other groups may be operative 

 in replication; and others in removing inhibitors. The probability of 

 effective damage by one ionization (or cluster) may be different from one 

 portion of a virus to another. For example, as mentioned later (see Sect. 

 3-2), phage particles inactivated by X rays (direct effect) fall into two 

 categories, some capable and some incapable of killing the bacterial host 

 (Watson, 1950). The two types are probably damaged in different parts 

 or in different ways. 



In the second place, even if there were a target within which an act of 

 absorption was always effective, there would be no reason for its geometric 

 interpretation as a sum of spheres. The observed dependence of radia- 



^ It has been stated that the discrepancies ahnost disappear for a particles (Bonet- 

 Maury, 1947, 1948); this seems doubtful, however, a-ray data include disturbing 

 contradictions such as a great disparity in sensitivity of vaccinia virus as measured by 

 different authors (Lea and Salaman, 1942; Bonet-Maury, 1947) and a target size 

 reported for poliomyelitis virus (Lansing strain) much larger than the generally 

 accepted virus size (Bonet-Maury, 1948). 



