GENETICS IN MODERN BIOLOGY — BEADLE 401 



Each time the material is replicated it doubles, so 20 replications rep- 

 resents more than a million copies. How does replication occur with 

 the precision necessary to avoid intolerable numbers of mistakes? 



How are the specifications — the directions or the recipe for making 

 us — translated ? This is an enormously difficult question, and I shall 

 say right now that we know very little about it. 



How are specifications modified during the course of evolution? 

 Most of us believe in organic evolution, and we want to know how 

 we have come to be different from our ancestors. In other words, 

 what is the nature of the mutation process? 



A few years ago, seven or eight years ago, we would have had a very 

 difficult time answering the four questions that I have just asked. We 

 did not know enough, and we did not have many good clues even as 

 to how we might go about searching for answers to these questions. 

 But within the past half-dozen years or so excellent clues have turned 

 up. In 1953, shortly before the M.I.T. Dorrance Laboratories of 

 biology and food technology were opened and dedicated, there oc- 

 curred an important turning point in modern biology. Wliat was it 

 and what does it have to do with answering the questions I have 

 posed ? 



By this time it had become quite clear to a number of biologists that 

 a particular chemical substance called deoxyribonucleic acid was im- 

 portant in transmitting hereditary information in bacteria and in 

 viruses. Since the cells of all higher plants and animals contain 

 deoxyribonucleic acid, it seemed probable that this substance served 

 to carry genetic specifications in all living systems. 



I shall attempt to explain how and why this substance, DNA for 

 short, is important. And I shall try to do it without considering the 

 details of its rather complex chemistry. DNA has been known for a 

 long time. And it was known to consist of long chainlike molecules 

 made of four kinds of units called nucleotides. But it was not known 

 exactly how DNA molecules were internally organized until 1953, 

 when two investigators — Dr. James D. Watson, now at Harvard 

 University, and Dr. Francis H. C. Crick of Cambridge University — 

 succeeded in formulating a structure that has proved to be substan- 

 tially correct. 



From the information then available from classical organic 

 chemistry, from X-ray diilraction studies, from analyses of the rela- 

 tive proportions of the four kinds of nucleotides, and through in- 

 genious model building, Watson and Crick proposed the structure 

 illustrated in figure 1. 



This Watson-Crick structure was at once exciting to the biologists. 

 Wliy? Because it suggested such plausible answers to the four 

 questions : How is genetic information written ? How is it replicated ? 

 How is it translated ? And how does it mutate ? 



