282 C. LEVINTHAL 



can be encompassed in a reasonably coherent and simple picture and that 

 the main effort will involve further attempts to understand the details of the 

 formal genetic schemes in terms of the physical chemistry of the DNA mole- 

 cule. On the other hand, there are others who see in some of the current 

 problems and apparent contradictions, the prospect of a breakdown of most 

 of the currently fashionable ideas and, possibly, even a new and fundamental 

 complementarity or uncertainty principle in biology. This chapter will be 

 written from the former pomt of view. It will also be \NTitten for readers who 

 are assumed to be unfamihar with genetics, microbial or otherwise; and in an 

 attempt to avoid discussions of terminology, the words "gene" and "chromo- 

 some" wiU not be used in connection with phage. 



II. Phage Mutations 



A. Plaque-Type Characters . 



If an excess of sensitive bacteria is mixed in a layer of nutrient agar with 

 a small number of phage particles, clear holes or plaques are formed in the 

 otherwise continuous layer of bacterial growth (see Adams (1950) for a dis- 

 cussion of currently employed techniques). If the phage particles in a single 

 plaque are used to inoculate a culture of sensitive bacteria, one can obtain a 

 stock of 10^^ to 10^2 particles per milliliter. Most of the plaques produced 

 when this stock is plated are identical with those used for inoculation; but 

 among several thousand, a few will differ in various ways from the majority 

 type. Some of the altered plaques are smaU, due to a chance delay in the 

 initial attachment of a phage particle to an indicator bacterium. If these are 

 picked and the phage they contain replated, the platings show the same 

 distribution of types that was shown by the original stock. However, an 

 occasional altered plaque is due to a hereditary change in the phage, as can 

 be demonstrated by the fact that the alteration continues to show itself when 

 phage from the plaque is replated. These observable changes in plaque mor- 

 phology which continue to appear after the modified phage has been grown 

 through several cycles in new bacteria are defined as genetic changes in the 



phage. 



The abihty to detect a particular genetic variation may depend on the 

 type of bacterial ceUs and the medium on which the phage is grown. How- 

 ever, from the point of view of formal genetics one need not consider the 

 nature of the physiological changes in the altered virus or the mechanism by 

 which the observable alteration arises. All that must be determined is that 

 the observable change in plaque morphology continues to show itseff w^hen 

 the progeny of the original altered phage are plated. The total number of 

 different observable changes that has been used m phage genetics is smaD, 



