66 NUCLEOPROTEINS AND NUCLEIC ACIDS 



purines, adenine and guanine, and two pyrimidines, cytosine and 

 thymine, occur in all deoxypentose nucleic acids, with the ex- 

 ception of certain E. coli phages in which cytosine is replaced 

 by 5-hydroxymethylcytosine. A minor, fifth component is often 

 encountered, viz., 5-methylcytosine. For the structure of the 

 nucleosides and nucleotides a recent survey may be consulted^^. 



The evidence accumulated thus far indicates that there exists 

 a very large number of different nucleic acids, which can be 

 distinguished by different proportions, and therefore by differ- 

 ences in the sequence, of the component nucleotides^. The two 

 most extreme examples are the deoxyribonucleic acid from a sea 

 urchin, in which the ratio of the sum of adenine and thymine to 

 that of guanine and cytosine ("the dissymmetry ratio") is 1.9 

 ("AT type"), and, on the other hand, the deoxyribonucleic acid 

 of the avian tubercle bacillus in which it is 0.4 ("GC type"). 

 While species specificity of composition can often be demon- 

 strated, no indications of organ specificity have as yet emerged. 



The incorrect tetranucleotide hypothesis has been abandoned; 

 and it must be concluded that the nucleic acids are complicated 

 and intricate high polymers, devoid of a recognizable periodicity. 

 Despite wide divergences in their composition all deoxyribonu- 

 cleic acid preparations appear to be characterized by several 

 regularities^ These are: (a) A + G = C + T; (b) A = T; (c) 

 G = C; and, therefore, (d) A + C = G + T*. Only the last 

 regularity, with thymine being replaced by uracil — A + C = 

 G + U — seems to apply to the ribonucleic acids^; in other 

 words, the molar sum of compounds carrying an amino group 

 in 6 position equals that of compounds with a 6-keto group in 

 both types of nucleic acid. 



4. FRACTIONATION OF DEOXYPENTOSE NUCLEIC ACIDS 



Throughout all our work on the composition of nucleic acids 

 we were beset by one question: does a nucleic acid preparation. 



* Abbreviations: A, adenine; G, guanine; C. cytosine; T. thymine; U, 

 uracil; or the corresponding nucleotides. 



