52 DEOXYPENTOSE NUCLEOPROTEINS AND PROSTHETIC GROUPS 



by the following regularities^' ^i* ^^i (a) The sum of the purine 

 nucleotides equals that of the pyrimidine nucleotides, (b) The 

 molar ratio of adenine to thymine equals 1. (c) The molar ratio 

 of guanine to cytosine (+ methylcytosine) equals 1. (d) The 

 number of 6-amino groups (adenine + cytosine + methylcyto- 

 sine) equals that of 6-keto groups (guanine + thymine). The last- 

 mentioned regularity, and in some cases also the others, but with 

 uracil taking the place of thymine, applies also to certain pentose 

 nucleoproteins^^. 



It is, in fact, because of these relationships that it is possible 

 to state that, for instance, in the deoxypentose nucleic acids of 

 wheat germ 5 -methylcytosine substitutes for a part of the 

 cytosine^^' 2^ or that in certain bacteriophage nucleic acids 5- 

 hydroxymethylcytosine replaces cytosine entirely^^. It is, in- 

 cidentally, remarkable that so far no purine satellites have been 

 discovered. 



But nature is inexorably approximate; and one could suspect 

 that occasional deviations from fit are more significant than the 

 fit itself. This may still turn out to be true of such giant molecules 

 as the nucleic acids, but I do not believe it will. In any event, I 

 should like to cite here a recent survey in which the results of 

 all published analyses, carried out in several laboratories, of 

 deoxypentose nucleic acid specimens from about fifty different 

 genera were tabulated^ The average values of the molar relation- 

 ships that bring out the complementariness principles mentioned 

 before were as follows. 



Adenine to thymine 1.009 



Guanine to cytosine (or its analogues) 1.001 



Purines to pyrimidines 1.000 



6-Amino to 6-keto groups 1.008 



If it is considered that the nucleic acids varied very widely in 

 their nucleotide composition (ratios of adenine + thymine to 

 guanine + cytosine between 2 and 0.4), the agreement of the 

 1 : 1 relationships is most impressive, especially in view of the 

 diversity of sources, preparations and procedures. No authentic 

 case of an appreciable deviation has yet come to my attention. 



