72 NUCLEOPROTEINS AND NUCLEIC ACIDS 



relation between specific sequences in the nucleic acid and 

 specific synthetic capacities has been established. 



(f) The fact that the nucleotide distribution in deoxypentose 

 nucleic acids is characteristic of the species, and unchanged in 

 different tissues of the same host, must be contrasted with the 

 far-reaching changes undergone by the proteins. 



But an old question must be asked before many of these ar- 

 guments: What is a cause, what is a symptom? Are nucleic acids 

 different because they come from different cells, or is this very 

 difference one of the causes of biological specificity? We seem to 

 have lost the ability to say "We don't know". But this is a 

 privilege that I still wish to retain. 



We may draw the following preliminary conclusions from the 

 work on the chemistry of the nucleic acids. There exists a very 

 large number of differently composed individuals which are 

 distinguished by differences in the sequence of their component 

 nucleotides; for we must remember that different composition 

 signifies different sequence. No perceptible periodicity, nor a 

 repeating unit, can be claimed; but the distribution does not ap- 

 pear to be random. In calf thymus deoxyribonucleic acid the 

 purine nucleotides have a greater chance to be next to, or near, 

 other purine nucleotides than next to pyrimidine nucleotides, and 

 the converse must be true of the latter^^. Similar conclusions can 

 be drawn from our older work on ribonucleic acids-^, when ac- 

 count is taken of what is now known about the specificity of 

 pancreatic ribonuclease-^. 



Yet, deoxyribonucleic acids, whether the whole or the sub- 

 fractions, all show the impressive regularities that I have men- 

 tioned before in Section 3; and most ribonucleic acids appear to 

 exhibit at least one regularity (Table 19). There may be some 

 that will ask whether there is any sense in chasing such regular- 

 ities or even whether, in view of the mischief worked by the now 

 defunct tetranucleotide hypothesis, a search for such norms is 

 not outright deplorable. If this represented an esthetic quest for 

 pre-established harmony or the music of the spheres, such ob- 

 jections would, perhaps, be justified. But quite apart from these 



