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think Dr. Chargaff has gone much further than this in the characterization of the 

 DNA molecules by extraction technics from the thymus gland. He has shown 

 that, depending upon the salt concentration, one can isolate several desoxynu- 

 cleic acids of high molecular weight of different composition. 



CHARGAFF: We have fractionated practically everything that you can 

 lay your hands on with the exception of phage where the fractionations are not 

 very easy, but you can get up to 10 fractions. 



more. 



CARTER: The chances are if you had the patience you would have 



CHARGAFF: I think there can be 100, 000. 



CARTER: This is extremely important. 



COHN: It looks like a continuous spectrum. 



TOBIAS: How are these differences characterized? 



CHARGAFF: By composition. There is a certain similarity which I 

 don't want to go into. But you have a spectrum which begins on one end with a 

 high guanine and cytosine content of the nucleic acid and goes to a very high 

 adenine and thymine and very low guanine and cytosine, and from the distribu- 

 tion curves you can figure out that you must have a very large number of indi- 

 viduals. 



CARTER: This actually lays very firm ground work for our interpre- 

 tation of data bearing upon structure of nucleic acids. I am sure that most 

 people in the room have never heard of Walter Jones, but he has contributed a 

 classical statement which probably will survive much longer than some of his 

 scientific contributions, and that is "a nucleic acid is a method of preparation." 



COHN: He said that about 1920, didn't he? 



CARTER: Way back. But this is being amplified continuously. 



CHARGAFF: There are all sorts of proteins. This is a very general 

 statement. 



COHN: There is an inconsistency in speaking of proteins as plural 

 substances and of RNA and DNA as single substances. This is often done rather 

 loosely in conversation. 



CARTER: At any rate, we have many factors which must be consid- 

 ered in the modification of the interpretation of these isotope incorporation data, 

 and perhaps now what we really must do is not only have the characterization of 

 these fractions but we must make all of our correlations in terms of biological 

 activity. At this stage of the game we cannot do that for most of the nucleic 

 acids. Where the activity brings about inheritable transformation in a few 

 select bacteria this can be done. It is obviously the most fruitful area in which 

 to study desoxynucleic acid, because the criterion of biological activity can be 

 employed in the study of the high molecular weight components. 



MAZIA: Except that it is a criterion of whether it is native or not, 

 but it can't possibly be, the way it is set up now, a criterion of purity. 



