CONCEPTUAL BASIS 129 



b. The problem of macromolecular structure 



It would, of course, be good if the investigator of the structure 

 of a deoxyribonucleic acid knew whether he was deaUng with one 

 chain or with two^^ or even more^- complementary chains; but at 

 the present, little advanced, state of our knowledge of the nucleic 

 acids this is not essential. The value of any information on the 

 sequential characteristics of the nucleic acid is not diminished if 

 the complementary chains are considered to be joined at one end 

 or even at both ends, as may well be the case, so as to constitute 

 an uninterrupted sequential progression. I am not aware of the 

 demonstration of distinct end groups in undegraded deoxy- 

 ribonucleic acid. 



The maintenance of sequence integrity of the preparations to 

 be examined is, on the other hand, a problem of great importance. 

 If — quite apart from the secondary valence forces supporting the 

 architecture of the secondary or tertiary structures of the deoxy- 

 ribonucleic acids — there really exist weak links in their primary 

 structure^^, it may be essential to avoid the rupture of the latter, 

 since otherwise valuable features of sequential arrangement may 

 be lost. This appUes, of course, even more to the avoidance of 

 chemical or enzymic degradation during the isolation. 



It is quite likely that very few of the deoxyribonucleic acid 

 preparations described so far, and none of the models proposed 

 to describe their structure, are representative of the native state. 

 The actual operative entity — though not necessarily amenable 

 to biological testing in vitro — possibly is an aggregate of very 

 long polynucleotide chains linked to each other, perhaps by 

 oUgopeptide bridges, and bonded to proteins in a spatially unique 

 configuration. If this is so, the problem of heterogeneity, men- 

 tioned in the preceding section, is ostensible rather than actual, 

 having been introduced as a necessary, but strictly non-biological, 

 artifact of purification. By this token, a preferred isolation 

 method for deoxyribonucleic acid would be one that avoided, as 

 far as possible, denaturation by chemical or physical means. 

 Nearest to these requirements is perhaps the procedure described 



References p. 159 



