102 NUCLEIC ACIDS AND BIOLOGICAL INFORMATION 



2. ON BIOLOGICAL INFORMATION CHEMICALLY CONVEYED 



A biochemist, when asked to consider this problem, probably 

 would first of all translate it into the concept of chemical speci- 

 ficity — with which I have dealt in more detail on several oc- 

 casions^-^ — and think of a substance of an elevated molecular 

 weight, built of a number of chemically different monomers and 

 possessing chemical or physical features which are preserved 

 without change within the species, but which serve to distinguish 

 the particular substance from analogous ones produced by other 

 species. Specific polysaccharides, specific proteins, specific 

 nucleic acids are the type of compounds that will come to mind 

 immediately; and the immunological specificity exhibited by 

 many representatives of the first two groups perhaps is their most 

 striking property. The recognition of the existence of specific 

 nucleic acids required much more time; but their specificity, too, 

 can now be taken to be an established fact on both chemical and 

 biological grounds^-^. It is, indeed, only in the fourth group of 

 principal cell constituents, namely the lipids, that difficulties with 

 respect to the occurrence of species specificity still are en- 

 countered. As I have pointed out recently^, one way in which 

 the cell may be able to render lipids specific is in their arrange- 

 ment as prosthetic groups of lipoproteins. But there exist in- 

 dications that nervous tissue contains complex mucolipids of high 

 molecular weight which could very well exhibit species speci- 

 ficity 6. 



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

 question: How identical is identity? In considering cell repUcation, 

 we hke to assume that the complex and specific cell constituents 

 are reproduced so as to give entirely invariant structures, with the 

 hundreds or even thousands of different amino acids or nucleo- 

 tides 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 



