THE PROBLEM OF NUCLEOTIDE SEQUENCE IN DEOXYRIBONUCLEIC ACIDS 77 



this survey is destined to be of great value. It permits the examination of 

 detailed features of the nucleotide arrangement, far beyond the possibilities 

 of distinction offered by total constituent analysis. I have selected, for 

 inclusion in Table I, three examples of nucleic acid fractions that cannot 

 be distinguished analytically. There can be little doubt about the entirely 

 different sequential plans governing the structure of these polymers if the 

 values for the abundance of solitary pyrimidine units are compared. 



NUCLEOTIDE ARRANGEMENT IN DIFFERENT DEOXYRIBONUCLEIC ACIDS OF 



ANIMAL ORIGIN 



Table II compares the distribution findings obtained with total, un- 

 fractionated preparations of the deoxyribonucleic acids from calf thymus, 

 human spleen and sperm preparations from two sea urchin species. These 

 data do not exhaust the information gathered by us on different animal 

 nucleic acids [21], but are presented here to indicate the scope of the 

 method. 



NUCLEOTIDE ARRANGEMENT IN THE TOTAL DEOXYRIBONUCLEIC ACID OF RYE 

 GERM AND IN A COMPLETE SERIES OF FRACTIONS 



The use of a deoxyribonucleic acid from a plant source offers several 

 advantages. These nucleic acids ha\ e been shown to contain a fifth nitro- 

 genous constituent, 5-methylcytosine, in appreciable quantity [7, 20]. In 

 the specimens isolated from both wheat and rye germ the concentration of 

 this pyrimidine amounts to 5 -9 mole % (Table III). This means that nearlv 

 a quarter of the cytosine has been " replaced" by methylcytosine. Since the 

 molar concentration of guanine equals the sum of these two pyrimidines, 

 one may consider them as pleromeric in the sense defined above. Minor 

 constituents of deoxyribonucleic acid (or, for that matter, of ribonucleic 

 acid) are destined to play an important role in future work on the elucida- 

 tion of nucleic acid structure, as they will serve as additional markers in 

 the array of nucleotides constituting the polymer chain. Furthermore, the 

 presence of a pyrimidine sharing with cytosine the property of being a 

 6-amino derivative and with thymine that of being a 5-methyl pyrimidine 

 is of the greatest interest for a better understanding of the mechanisms of 

 selection, incorporation, and replication that are at work. 



Before the study of the nucleotide arrangement in the deoxyribonucleic 

 acid of rye germ could be undertaken, it was necessarv to examine the 

 properties of deoxymethylcytidine 3 ',5 '-diphosphate whose production is 

 indicative of the frequency of this pyrimidine as a solitary nucleotide [39]. 

 At the same time the two dinucleoside triphosphates comprising 5-methyl- 

 cytidine and either cytidine or thymidine were also studied. 



