FRACTIONATION OF DEOXYPENTOSE NUCLEIC ACIDS 67 



as it is isolated from a given cell, represent one molecular species, 

 several or many? On the one hand, we had observed a remark- 

 able constancy in composition, at any rate of the deoxypentose 

 nucleic acids from the same species. On the other hand, it ap- 

 peared unlikely on general biological grounds that a cell gave rise 

 to only one kind of deoxypentose nucleic acid. But the task of 

 separating a mixture of closely related polyampholytes of a very 

 high molecular weight appeared hopeless. Often, when I discussed 

 this problem with my colleagues, I was asked what could be done 

 about it; and my answer was that, if we were dealing with three 

 or four different nucleic acids, a separation may be feasible, but 

 that with a very large number of individuals in the same prepa- 

 ration we might as well treat them as one compound. This is, 

 indeed, a problem that leads us directly into very deep waters, 

 namely, the meaning of chemical identity in macromolecules. 



In any event, it occurred to us that a separation, even if partial, 

 could perhaps be effected at the nucleoprotein stage. As I 

 mentioned before, the fractional dissociation of nucleohistone by 

 salt solutions of increasing concentration seemed to offer pos- 

 sibilities. This hope was realized^^, and the findings were later 

 extended in our laboratory^^- ^^' ^^, and confirmed in others^^* ^'^. 

 (For a survey, compare Ref. 3). As an example, I am showing 

 here the fractionation of a preparation of wheat germ nucleo- 

 protein which is described in a paper that is in course of 

 publication^^. The data are assembled in Table 18. The typical 

 picture can be seen: the fractions are characterized by progres- 

 sively decreasing proportions of guanine, cytosine and 5-methyl- 

 cytosine and progressively increasing quantities of adenine and 

 thymine; but the unity relationships are maintained. There is 

 noted, moreover, an uneven distribution of methylcytosine with 

 respect to cytosine. 



Numerous fractionation experiments with different nucleic 

 acids, in many of which artificially prepared complexes with 

 basic proteins^^' ^^ or polyelectrolytes^^ were employed, have led us 

 to conclude that a preparation of deoxyribonucleic acid may com- 

 prise a very large number of differently constituted individuals. 



References p. 75 



