PEOBLEMS OF MEASUREMENT OF MUTATION RATES 63 



it had been a time-dependent phenomenon, but, in fact, when one 

 made allowance for the 30 per cent increase in DNA in the very early 

 part of the starvation period, then there was no further increase in 

 the number of mutants, which would suggest, again, in this particular 

 system, continued DNA replication was essential for the continued 

 production of mutations. 



Zamenhoj: Perhaps we could agree on what is actually called mu- 

 tation. Suppose there is a mutagenic process, and then the cells are 

 stored without cell division. Finally the cells divide, and one detects 

 mutation; which of these steps, or all three, should be called mutation? 

 In the first process, some change is registered in DNA. To give an 

 example we heated the cells to 155° which must have registered some 

 change in DNA (43), then we stored the cells for a month, and finally 

 we let them divide, whereupon the mutation was discovered. Now, 

 which of these processes should be called a mutational process? 



Auerhach: The first. 



Benzer: Why? As long as you are unable to separate them, how 

 can you assign a developmental role to one or the other? 



Auerhach: If you can correlate the frequency of the final event 

 with the type of treatment you use, then the primary step must be 

 due to this treatment, and one can call that mutation. 



Zamenhof: Because the second step might be quite trivial. 



Auerhach: What I call the primary step in mutation must be due to 

 the treatment. 



Benzer: But this distinction could come only as a result of investi- 

 gation. The mere observation does not enable you to project into it 

 your own ideas as to the stage of the primary event. 



Auerhach: No, whether the actual change in base sequence is in- 

 duced at this time or not, we cannot say, but whatever the chain of 

 events is afterwards, if the same final event is always correlated with 

 the same treatment, then the essential process must have occurred 

 during treatment. 



Goodgal: No, that doesn't follow, because if there are a number 

 of different steps in the process of mutation, then you may be able 

 to induce mutation at every one of them. You can't say anything until 

 you define the steps. 



Zamenhof: But the second may be trivial. Cell division may always 

 be the same, and in that case it is not the rate-determining step, as 

 Dr. Atwood said. The rate-determining step would be actually the 

 first treatment with mutagen. This step decides what the final out- 

 come will be. 



