DEOXYPENTOSE NUCLEIC ACIDS 57 



complete replacement of cytosine by 5-hydroxymethylcytosine in 

 certain phage nucleic acids-^. The latter synthesis, at any rate, 

 may be presumed to occur in spite of the presence of the anal- 

 ogous pyrimidine, cytosine. 



e. Is there more than one deoxypentose nucleic acid in a cell? 



An answer to this question appeared very difficult for a long 

 time. On the one hand, many different nucleic acid preparations 

 from the same genus showed a remarkable constancy of com- 

 position. On the other hand, it appeared rather unlikely on 

 general grounds that a cell, which is able to produce so many 

 different proteins, should give rise to no more than one deoxy- 

 pentose nucleic acid. But the task of separating a mixture of 

 closely related polyampholytes of a very high molecular weight 

 appeared hopeless; and one could ask whether a nucleic acid 

 preparation comprising a very large number of individuals should 

 not be treated as one compound. This is, indeed, a problem that 

 leads directly into the epistemology of chemical identity; and 

 many have found that it is an ungrateful task to spend one's life 

 at the frontier of knowledge without the proper passport. 



Later, a process for the fractionation of deoxypentose nucleic 

 acids was discovered which is based on the fractional disso- 

 ciation of a nucleoprotein, as discussed above (see also Fig. 2). 

 The procedure, which led to the separation of many deoxypentose 

 nucleate preparations into a series of fractions of divergent 

 purine and pyrimidine contents, was first applied to calf thymus 

 nucleohistone^ and later extended to the fractionation of other 

 nucleic acids through their complexes with histone, globin^ or 

 polylysine^^. Subsequently, a similar arrangement was described 

 in a preliminary form in which the fractionation was carried out 

 by means of a histone-kieselguhr column^. 



The gradual extraction of nucleohistone or of an artificially 

 prepared protein nucleate with salt solutions of increasing strength 

 in the presence of a denaturing phase, such as chloroform-pen- 

 tanol, was found to yield a series of nucleic acid fractions with 

 diminishing concentrations of guanine and cytosine and rising 



References p. 60 



