Virus Action and Replication 127 



different animal viruses in cell cultures have been developed. This 

 approach lias, for the first time, allowed animal virology to be 

 studied at the cellular level with homogenous populations of cells 

 which can be simultaneously infected (30). 



It is apparent that animal viruses present a vastly less homo- 

 genous group of agents tlian bacteriophages. The host range in- 

 cludes virtually all animals from single celled organisms to verte- 

 brates. These viruses range in size froml50A to 3000A; they are 

 of many different shapes and show great variability in chemical 

 composition. Moreover, the host cell for an animal virus is con- 

 siderably more complex than a bacterial cell. It is, tlierefore, not 

 surprising that the interrelationships between animal viruses and 

 their host cells are quite varied and complex. This discussion will 

 center on one animal virus-cell system, the poliovirus infected 

 HeLa cell, which is one of the most thoroughly studied animal 

 virus systems. 



First, a few of the chemical and physical properties of polio- 

 virus should be stated (31). It is a small (300 A in diameter) 

 so-called spherical virus which is composed of RNA, 25 per cent, 

 and protein, 75 per cent. The RNA is enclosed within a protein 

 shell made of subunits which are arranged in a symmetrical form 

 (icosahedral) on the particle surface (32, 33). There are no lipids 

 or cell-derived macromolecules attached to the virus particle (31). 

 The infectious-unit-to-particle ratio is low (of the order of 1 in 100) 

 in both crude and purified virus suspensions (31). 



The initial event of infection with poliovirus in a normally 

 susceptible cell is adsorption, which is, at least to some extent, 

 dependent on the ionic strength of the medium and on the presence 

 of divalent cations (34, 35). After adsorption the infectious virus 

 disappears rapidly from the surface of the cell. It can be shown 

 with virus labeled by P'^" in its RNA that all particles in a purified 

 suspension can adsorb to the cell but that about 50 per cent of 

 these come back off the cell in a non-infectious and non-adsorbable 

 state (36). The extracted RNA in these particles is still as infec- 

 tious as in an unexposed suspension, however. The RNA of most 

 of the particles which do remain attached to the cell is degraded 

 to small pieces. About 10 per cent of the attached particles remain 

 unchanged and about 10 per cent are changed in such a way that 



