4 THE PHYSICS OF VIRUSES 



size are to some extent dependent on the previous history of 

 the virus. A virus which has been dried and heat treated has a 

 longer latent period and a smaller burst size. 



Various agents can be used to estimate the number of virus 

 particles present at any stage of this process. The larger phages 

 can be seen as scattering centers in the ordinary microscope. 

 The number of these drops from one, at the original moment of 

 invasion, to zero for the first 95% of the latent period. In the 

 last 5%, many scattering centers are seen, and these correspond 

 to the number of virus particles released. Or the cellular processes 

 can be stopped with proflavin, the cells forcibly burst open by 

 pressure, and the contents examined with the electron micro- 

 scope. The results of this kind of study show that, at about 

 85 % of the latent period, incomplete virus particles are present — 

 doughnut heads in place of barrel-shaped heads, for example. 

 Thus, virus assembly would appear to be a relatively late and 

 correspondingly rapid process. Similar conclusions can be 

 reached by bombarding infected E. coli with X-rays at various 

 stages of the latent period. Only toward the end of the latent 

 period does a multiple-hit type of survival curve become appar- 

 ent, again indicating that virus assembly is a late process. This 

 process of assembly is not understood — the reader must see that 

 it represents a tremendous intellectual challenge. 



Size and Shape of Viruses 



This will be the subject of detailed consideration later. The 

 shortened list in the table on page 5 gives an idea of the range 

 of sizes covered by viruses. The unit used is the Angstrom, 10~^ 

 cm, which is the experimental unit used in describing atomic 

 and molecular sizes. 



The objects we deal with, therefore, vary in diameter over a 

 factor of 45, and in mass by a factor of roughly 10^ One might, 

 therefore, be on guard regarding treating viruses as all alike, 

 and no attempt will be made to do so here. 



Not nearly enough is known of the true shape of viruses. They 

 seem to be constructed, very broadly speaking, out of spheres 

 and rods. Only for four plant viruses — tobacco mosaic, southern 



