Nucleic Acids as Carriers of Biological Information 301 



the constituent nucleotides in different specimens, has, since the time of its first 

 discussion [i, 12], been demonstrated in very many instances [2]. Of more recent 

 date is the discovery that a total DNA preparation can be fractionated into a 

 whole series of differently composed, but regularly graded, fractions [13]. The 

 distribution of these fractions in a given cell must, however, be constant within 

 narrow limits, as shown by the invariability of total DNA mentioned above. 



The variation in the composition of different PNA preparations is marked, 

 though perhaps not quite as striking as in DNA [10]. In addition, there is some 

 evidence of the existence of different pentose nucleic acids in the same cell; 

 e.g., the nuclear and the cytoplasmic PNA of rat Uver differ in composition [14]. 

 A reliable chemical fractionation of PNA has, however, not yet been achieved. 



REGULARITY OF NUCLEIC ACIDS 



As I have mentioned before, this is perhaps the most unusual property dis- 

 tinguishing the nucleic acids from other cell-specific high polymers. In saying 

 this, I am not referring to the regular position of the phosphodiester bridges 

 connecting the nucleosides in the nucleic acid chain: they are assumed to be 

 generally 3' : 5 '-bridges; for the regularity of the Hnks holding the monomers 

 together is common to most biological polymers. What is so pecuHar is the 

 remarkable balance between the several constituents noticed in all deoxypentose 

 and in almost all pentose nucleic acids: a type of equipoise that I am not aware 

 of ever having been encountered in other mixed polymers that do not contain 

 simple repeating units. 



There are several regularities of which three are characteristic of only DNA 

 [i, 2]. They are: (i) the molar quantity of adenine equals that of thymine. 



(2) The molar quantity of guanine equals that of cytosine (+ methylcytosine). 



(3) The sum of the purine nucleotides equals that of the pyrimidine nucleotides. 

 The fourth regularity, finally, applies to nearly all nucleic acids, DNA and 

 PNA [10]; it is: (4) The molar sum of nucleotides carrying 6-aniino groups 

 (adenylic, cytidylic acids) equals that of nucleotides having 6-keto groups 

 (guanylic, thymidyHc or uridylic acids). 



In considering these regularities one must take into account that the nucleic 

 acids are devoid of any perceptible periodicity or of repeating sub-units larger 

 than a mononucleotide. They are compHcated high polymers of a largely 

 arrhythmic nucleotide sequence which, however, does not appear to be 

 fortmtous [15]. 



CONCLUDING REMARKS 



It is inviting to assume that the special biological functions of the nucleic 

 acids are reflected in those chemical features that distinguish them from other 

 high-molecular cell components, namely, in the unusual regularities in nucleo- 

 tide composition mentioned above. In which way these functions are exerted 

 cannot yet be said; but I already have pointed out before that in my opinion the 

 nucleoproteins rather than the separate moieties of the conjugated proteins will 

 eventually be found to be the operative units. In this connection, reference may 



