13 



The Summing Up 



In writing this book we have tried to include within each chapter 

 not only a summary of the current knowledge, presented in terms of 

 illustrative experiments, but also a discussion of areas of uncertainty 

 and unsolved problems. In any developing science, one is confronted 

 with the array of past achievements which have molded the science as it 

 is known today, and simultaneously with the areas of ignorance which 

 still exist. It is not easy to present a balanced view. Often, in text- 

 books, problem areas are consigned primarily to the last chapter. Since 

 we have considered unsolved problems throughout the book, our purpose 

 in this last chapter is of a different nature. We shall attempt a very 

 brief general view of the present state of cellular genetics, in respect 

 to the science itself and to its impact upon certain other areas of learning 

 and of everyday life. 



At the outset, we proposed to consider cellular heredity in terms of 

 the nature of hereditary materials, the analysis of transmission mechan- 

 isms, and of the translation systems by means of which control is exerted 

 over metabolism and development. It seems appropriate, then, to sum- 

 marize the book with reference to these categories. 



NATURE OF HEREDITARY MATERIALS 



Nucleic acids have been directly identified as carriers of genetic in- 

 formation in bacterial and viral systems, and a great diversity of indirect 

 experiments support the hypothesis that nucleic acids serve this function 

 in all organisms. Most of the available information concerns DNA, 

 which is the active principal in bacterial transformation. The demon- 

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