202 THE PHYSICS of vihuses 



We need now to consider how the assembly of viruses from 

 moderately large molecules can take place. 



Forces Operative in Virus Multiplication 



We have seen how the process of virus multiplication involves 

 a dramatic change in the manufacturing processes of the bac- 

 terium so that the actual small pieces themselves are different, as 

 exemplified by the virus antigens, which are small yet diflFer from 

 the bacterial antigens. Although this is a most important fact, 

 there is the equally important fact that in some way a virus, or 

 part of a virus, can grow until it either divides or is the equiva- 

 lent of two viruses. This requires an ordered groicth, and the 

 ordering process requires some kind of forces to be acting. We 

 propose to consider here the kinds of forces which can be called 

 on to explain ordered growth, and to make a speculative sug- 

 gestion as to how they act. 



There are essentially two ways in which an ordered array of 

 matter can be made to develop. One is by the establishment of a 

 concentration gradient of some kind, and subsequent diffusion. 

 Thus if a cell is filled with small units of nucleic acid, and these 

 are condensed into a solid phase at some point in the cell, there 

 will be established a concentration gradient toward that ])oint, 

 and diffusion will occur. This diffusion will produce a steady 

 accretion of nucleic acid at the condensation point. There is no 

 doubt that this is an important feature of many cellular proc- 

 esses. However, the measured diffusion constants of protein 

 molecules indicate that this jjrocess is far too slow to account for 

 the very rapid virus multiplication. Accordingly, we turn to the 

 second way, which requires the action of forces. 



All forces between molecules are electrical in character, if we 

 exclude very weak forces. They have their origin in the Coulomb 

 field of a charged particle and owe their nature to the distribu- 

 tion of charge in the two systems which attract or repel each 

 other. We have already seen that the first stage in virus attach- 

 ment is electrostatic and must be due to the attraction of charges 

 of opposite sign. If two elementary charges are involved, and 

 they are separated by a distance /• cm in a medium of dielectric 



