ISOLATION AND COMPOSITION OF DEOXYPENTOSE NUCLEIC ACIDS 371 



is most impressive, especially in view of the diversity of sources, prepara- 

 tions, and procedures. 



An attempt has been made-^^ to derive a molecular structure of deoxy- 

 pentose nucleic acids from these regularities and from the available X-ray 

 evidence. [Compare the discussion in Chapter 13.] This hypothesis, which 

 postulates a helical dyad in which the two coiled strands are held together 

 by a specific pairing of the bases showing the unity relationships mentioned 

 above, has much to recommend itself on aesthetical grounds; it makes good 

 use of several experimentally established facts. Whether it does more than 

 to describe the structure of that portion of the processed preparation from 

 which the diffraction patterns are obtained, remains, however, to be estab- 

 lished. It is not improbable that the scheme is incomplete in some essential 

 features, at least insofar as substitution in position 5 of the pyrimidines is 

 concerned. If 5-methylcytosine or analogues could take the place of cyto- 

 sine and vice versa without restriction, the 6-amino pyrimidines should be 

 able to replace each other at random. This is obviously not the case, as 

 shown by the remarkable constancy of the 5-methylcytosine content of a 

 given species. (See, especially. Table XI.) The absence of uracil from de- 

 oxypentose nucleic acids also is not easy to understand on these grounds. 

 Even more disturbing, perhaps, is the fact, pointed out before (Section 

 VIII. 2 and Table XVI), that 5-methylcytosine is distributed unevenly in 

 the fractions obtained by the fractionation of calf thymus deoxyribonucleic 

 acid. One gains the impression that^ — ^just as certain phage nucleic acids 

 contain 5-hydroxymethylcytosine in total replacement of cytosine — there 

 exist nucleic acid molecules as part of the preparations from calf thymus or 

 from wheat germ in which cytosine is entirely replaced by 5-methylcyto- 

 sine, whereas other fractions are, in turn, completely devoid of the latter. 



3. A Concluding Remark 



It may be considered as intellectually quite unsatisfactory that a con- 

 siderable part of what is known about the composition and structure of 

 nucleic acids must, as has been shown here, rest on assiduous analytical 

 work. In our time, much stress is laid on the forces that govern the life and 

 the economy of the cell. The discovery that the cell runs while it rests has 

 effaced the other half of the truth: that it rests while it runs. The quiet 

 center is falling into oblivion ; and there is widespread contempt for what is 

 regarded as morphology or analysis. We should, however, comprehend that 

 it is by way of decimals that we penetrate into nature. We read in The 

 Wisdom of Solomon (11:21): "But thou hast ordered all things in measure 

 and number and weight." We can only hope that the span will not be too 

 wide, the count too high, the weight too heavy. 



