106 NUCLEIC ACIDS AND BIOLOGICAL INFORMATION 



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 prepa- 

 rations is marked, though perhaps not quite as striking as in 

 DNA^^. 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 liver differ in composition^^. A 

 reliable chemical fractionation of PNA has, however, not yet 

 been achieved. 



5. REGULARITY OF NUCLEIC ACIDS 



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

 property distinguishing 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 nucleo- 

 sides in the nucleic acid chain: they are assumed to be generally 

 3 ',5 '-bridges; for the regularity of the links holding the monomers 

 together is common to most biological polymers. What is so 

 peculiar is the remarkable balance between the several constitu- 

 ents 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^'^. They are: (1) 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^O; it is: (4) The molar sum of nucleotides carrying 6-amino 

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

 6-keto groups (guanylic, thymidylic or uridylic acids). 



In considering these regularities one must take into account that 

 the nucleic acids are devoid of any perceptible periodicity or of 



