THE CHEMICAL BASIS OF HEREDITY 3 



toward description on a molecular level, toward a science in which the 

 events of heredity can be described in terms of physics and chemistry, 

 and toward a physical concept of the gene that will allow prediction of 

 its behavior. 



It should be emphasized, however, that genetics cannot yet be pre- 

 sented as a molecular science. It is a science in transition, from the 

 purely biological to the physicochemical level of analysis, with some areas 

 more advanced in this direction than others. Our choice of illustrative 

 material has been weighted heavily toward experiments interpretable 

 on a physicochemical basis. Frequently these are very recent experi- 

 ments. Consequently, we have been led to consider current problems in 

 genetics much more extensively than past achievements. In this sense, 

 this book is an introduction to research areas in cellular genetics. 



IDENTIFICATION OF HEREDITARY MATERIAL 



The single most illuminating and significant advance in modern 

 genetics to date has been the chemical identification of a class of macro- 

 molecules — the nucleic acids — as hereditary determinants. This finding 

 has ramifications and implications which we shall be considering in 

 various contexts throughout the book. One important consequence has 

 been the opportunity of experimenting directly with hereditary materials 

 instead of relying solely upon inferences from indirect observation. 



In this chapter we shall examine the experimental evidence that hered- 

 itary determinants have indeed been identified; and we shall discuss some 

 of their properties. 



Formal methodology 



Consider two bacterial cells which differ by a single observable char- 

 acteristic: one is colored (a"*") and the other is colorless (a~) when grown 

 in the same constant environment. Each cell grows and then divides into 

 two daughter cells which, in turn, repeat the process and, after n divisions, 

 there are 2" progeny, which arose from a single parental cell; together 

 they form a clone. On inspection, one finds further that all cells of the 

 a"*" clone (which came from the a'^ parental cell) are a"*", and all cells of 

 the a^ clone are a'. Thus the color characteristic is a hereditary prop- 

 erty of the organism and is regularly transmitted to all progeny. 



Suppose that one makes various chemical extracts of the a clone. If 

 one treats a^ cells with extracts from a^ cells, can one transform a to 

 a"*"? Here we must distinguish carefully between (1) a physiological 



