DEOXYPENTOSE NUCLEIC ACIDS 51 



the survey in Ref. 1). To mention only the most extreme examples: 

 the ratio of the sum of adenine and thymine to that of guanine 

 and cytosine is about 1 .9 in the deoxyribonucleic acid of Echino- 

 cardium cordatum, but about 0.4 in that of avian tubercle bacilli. 

 It is, in fact, possible to make a gross distinction between three 

 types of deoxypentose nucleic acid: (a) the AT type, in which 

 adenine and thymine are the major constituents; (b) an inter- 

 mediate type, in which all bases are present in nearly equimolar 

 proportions (found only in Escherichia coli); and (c) the GC 

 type, in which guanine and cytosine predominate. While, therefore, 

 the question whether there exists more than one deoxypentose 

 nucleic acid must be answered in the affirmative, a decision 

 whether different tissues of the same host yield entirely iden- 

 tical nucleic acid specimens is more difficult. For the moment, 

 it would appear that differences in the composition of prepara- 

 tions from different organs, if they exist at all, are of very 

 doubtful significance. 



b. Do there exist recognizable repeating units? 



The tetranucleotide hypothesis is incorrect. One must conclude 

 that there is no subunit of recognizably recurrent structure larger 

 than a mononucleotide. If repeating polynucleotide units occur 

 in the nucleic acid chain, we have no way as yet of demonstrating 

 their existence. The nucleic acids must be regarded as complicated 

 and intricate high polymers of undetermined and largely ar- 

 rhythmic nucleotide sequence. 



c. Regularities 



The mistakes of the present are made by trying to avoid the 

 mistakes of the past. Though the danger of the premature im- 

 position of regularities is well exemplified by the tetranucleotide 

 hypothesis, it would be an error not to draw generahzing con- 

 clusions from experimentally established facts. The apodictic 

 manner of the following enumeration is chosen to conserve space. 

 Despite wide divergences in their nucleotide composition and 

 sequence all deoxyribonucleic acids appear to be characterized 



References p. 60 



