Fig. 2. Virus structure. The tobacco mosaic virus is made up from 

 at least two components. There is a central rod of nucleic acid and 

 a series of units of protein that fit over the central rod. In the 

 photograph shown here part of the second component has been 

 dissolved away to reveal the nucleic acid. (This photograph was 

 obtained through the kindness of Professor G. Schramm.) 



