550 S. E. LURIA 



to cell multiplication. Tlie thesis of this chapter will be that virus multi- 

 plication as a biological process belongs on the level of the replication of 

 subcellular elements, that is, on the level of cell growth rather than of cell 

 multiplication. 



In the same way as the morphogenesis of multicellular organisms must be 

 interpreted in terms of the creation of organized patterns of specifically 

 differentiated cells, so must the growth of cells be interpreted in terms of the 

 formation and maintenance of organized patterns of specific molecules and 

 macromolecular complexes. The morphogenesis of these patterns and the 

 synthesis of their constituent parts are the subject matter of cytochemistry. 

 The study of virus multiplication is a branch of cytochemistry; a remarkable 

 branch, in fact, since it presents unique opportunities for the study of some 

 cellular constituents in isolation in a fully native, functional, undegraded 

 form, and of their transition from the isolated, inert state to the integrated, 

 functional state as parts of the living protoplasm. 



That virus multiplication is not a process homologous to cell multiplication 

 is suggested immediately by the structure and composition of virus particles. 

 All cells caj^able of multiplication, no matter how different their origin, size, 

 and structure, contain certain essential chemical constituents — including 

 proteins and nucleic acids both of the ribose (RNA) and deoxyribose (DNA) 

 types — and certain essential organelles — nucleus, mitochondria, microsomes, 

 cell membranes. Instead, as discussed in Chapters 3 and 6, most virus 

 particles lack one or more of the basic chemical constituents of cells. 

 Their composition and organization are much simpler than those of any 

 cell. 



There is great variation in these respects among different groups of viruses. 

 Viruses are grouped together taxonomically on methodological rather than 

 biological criteria. There is no reason to assume that they represent a natur- 

 ally related group. The ability to invade living ceUs from outside and to 

 multiply within them, which is a major criterion used to define viruses, may 

 well be common to a variety of unrelated elements. The size of virus particles, 

 another criterion used to group viruses together, ranges over a factor of 

 10,000 in mass and is no indication of natural relationship. 



Whatever basic similarities exist among all viruses or among groups of 

 them can be revealed only by the methods of cytochemistry, that is, by the 

 study of the structure and composition of the virus particles and of the 

 chemical and physiological events that their presence and midtiplication 

 produce in the cells. The relevant facts are discussed in detail in other chapters 

 of this book. Here we are concerned only with tracing basic similarities and 

 outstanding differences among viruses as revealed in their processes of multi- 

 plication and with deciding whether any generalizations appear justified by 

 our present knowledge of these processes. 



