Nucleic Acids as Carriers of Biological Information 299 



A warning is, however, in order. We cannot yet answer the question: How 

 identical is identity ? In considering cell replication, we like to assume that the 

 complex and specific cell constituents are reproduced so as to give entirely 

 invariant structures, with the himdreds or even thousands of different amino 

 acids or nucleotides always arranged in an unchanged sequence. Actually, very 

 little is known; the extent of play that nature may allow itself cannot yet be 

 gauged. 



It is, in any event, safe to assume that the real specificity of a given cell resides 

 in the nature of its plastic constituents rather than in that of its catalytic com- 

 ponents, which latter probably are a symptom rather than a cause. But there is 

 a giant step from the estabhshment of sequential specificity in proteins, nucleic 

 acids, polysaccharides to the formulation of the manner in which the 'informa- 

 tion' stored in them is not only preserved, but also transferred from one class 

 of components to the other. The polysaccharides and lipids caimot at all be 

 fitted into what little we know. As regards the nucleic acids and proteins, how- 

 ever, it would appear to me safer, instead of postulating a hierarchy, to envisage 

 a triangular relationship, e.g., as follows : 



I 

 DNA 



/ \ 

 PNA Protein 



2 3 



No arrows have been placed in this diagram, for their direction may conceivably 

 be different in different systems; so that, for instance, in a plant virus the re- 

 lation is between 2 and 3, in bacteriophage dupHcation and in bacterial trans- 

 formations between i and 3, in autarkic systems between i, 2 and 3. It is, 

 moreover, quite conceivable that the transfer of information often can proceed 

 in either direction and that the prevailing impression that it invariably flows 

 from the nucleic acids to the proteins is mistaken. We may not yet have learned 

 how to isolate suitable protein preparations ; I should not be surprised if it were 

 found eventually that both halves of a plant virus, the PNA and the protein, 

 may be 'infective'. It was, therefore, not without purpose that I used the plural 

 above in speaking of 'life and its origins'. 



One could, however, ask : Is the cell really nothing but a system of ingenious 

 stamping presses, stencilling its way from life to death ? Is Hfe itself only an 

 intricate chain of templates and catalysts and products ? My answer to these 

 and many similar questions would be No; for I believe that our science has 

 become too mechanomorphic, that we talk in metaphors in order to conceal 

 our ignorance, and that there are categories in biochemistry for which we lack 

 even a proper notation, let alone an idea of their outlines and dimensions. 



Regardless, however, of whether we accept the currently fashionable template 

 hypothesis, we must assume that there exist agents or complexes of agents in 

 the cell that preside over the selection and specific arrangement of the consti- 

 tuents of all cell-specific polymers : proteins, nucleic acids, blood-group sub- 

 stances, bacterial polysaccharide antigens, etc. These agents must be able botli 

 to preserve and to transfer whatever codes are stored in the constituent sequence 

 and the specific physical shapes of these compounds. How these agents operate 



