104 NUCLEIC ACIDS AND BIOLOGICAL INFORMATION 



try 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 fash- 

 ionable 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 constituents of all cell- 

 specific polymers: proteins, nucleic acids, blood group substances, 

 bacterial polysaccharide antigens, etc. These agents must be able 

 both 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 is not yet within our 

 power to describe; but there exists some evidence that the nucleic 

 acids, both DNA and PNA, are concerned in these operations. 

 It lies within the scope of these remarks to inquire whether there 

 are any outstanding features that are common to all nucleic acids, 

 for this may be of value for our understanding of the chemical 

 characteristics of such information systems. In dividing the rest 

 of the discussion under the headings of invariability, diversity, 

 and regularity, it will be apparent that the first two quahties 

 probably are common to all specific cell constituents, whereas the 

 third is a most unusual feature of the nucleic acids which ap- 

 parently is not shared by the other plastic cell substances. 



3. INVARIABILITY OF NUCLEIC ACIDS 



This is a property that is, often by tenet rather than by experi- 

 ence, held to be common to all macromolecular cell components. 

 How limited our knowledge is, I have already emphasized before; 

 but it is, in general, assumed that a given protein or specific 

 polysaccharide or nucleic acid is invariable in structure in a 

 particular species. Although, as I shall mention below, there 

 exists impressive evidence that the nucleic acids of a cell com- 

 prise an entire series of different individuals, the constancy, 

 with respect to characteristic base composition, of total deoxy- 

 pentose nucleic acids appears well established^; a constancy that 

 seems to apply to preparations from all tissues of the same 



