17 



CHAPTER II 



THE SIZE AND SHAPE OP VIRUSES 



In "Viruses as Molecules" it was seen that purified virus preparations 

 can "be obtained by chemical and physical means. Such preparations can be used 

 to determine something of the nature of the virus particle. One of the first 

 things we all want to know about a virus is its size and shape. Numerous 

 physical methods are available for providing such Information. They include 

 ultrafiltration, ultracentrif ugation, diffusion, viscosity, streaua double re- 

 fraction, x-ray diffraction, and electron microscopy. It would be impossible 

 in the limited space available to examine the evidence on the size and shape of 

 all of the viruses that have been investigated thus far. Bather than attempt 

 this, it would be preferable to consider four viruses in more or less detail. 

 The four chosen are tomato bushy stunt, southern bean mosaic, tobacco mosaic 

 and Influenza viruses. 



Tomato Bushy Stunt Virus 



Let us see first how the tools of the physical chemist can be applied to 

 the study of tomato bushy stunt virus. The most spectacular tool for the in- 

 vestigation of viruses is the electron microscope. As was mentioned previously, 

 the electron microscope is a device which resembles the optical microscope in 

 principle, but which uses a streaun of electrons Instead of a beam of light and 

 magnetic or electric fields instead of lenses. Because of the short effective 

 wave length of the stream of electrons, the lower limit of resolution with the 

 electron microscope is far below that with the ordinary microscope. Electron 

 images are shadows. Matter scatters electrons. Those electrons scattered by 

 an object do not reach the photographic plate. Therefore, a shadow image of 

 the object is formed. The efficiency of matter in the scattering of electrons 

 depends upon its density. Viruses have low density and therefore a low scat- 

 tering power. For that reason their images have poor contrast. A technique 

 has been developed to improve the contrast. A very thin film of a heavy metal, 

 such as gold, is evaporated onto the virus particles at an angle. Then the 

 particles are micrographed . The effect is to produce images of particles with 

 shadows comparable to the effects produced by photographing landscapes in ob- 

 lique illumination. Figure 8 is an electron micrograph of bushy stunt virus 

 obtained by the shadow technique. It can be seen that the particles are ap- 

 proximately spherical bodies. The size has been determined as about 25 milli- 

 microns . 



More detailed information can be obtained by other physical methods. Among 

 these, the most important is the ultracentrifuge . Let us recall momentarily 

 what happens to a virus particle in a centrifugation experiment. When a parti- 

 cle moves through a viscous medium in a centrifugal field, it is subjected to 

 two forces - a centrifugal force, Fq , tending to increase its speed, and a 

 frictional force, F^, tending to decrease its speed. The particle moves at a 

 uniform rate when these two forces exactly balance each other. Now, the first 

 of these is a function of the magnitude, g, of the centrifugal field and the 

 effective mass, (m^-m ), of the particle, whereas the second is a function of 

 speed, V, with which ^ the particle moves, and the coefficient of friction, f, 

 of the particle. Since, according to Stoke's law, this friction coefficient 

 is directly proportional to the radius of a spherical particle, it is relatively 

 simple to calculate the size of a spherical particle from its sedimentation rate 

 in a known centrifugal field. These ideas can be summarized i 



