140 NUCLEOTIDE SEQUENCE IN DEOXYRIBONUCLEIC ACIDS 



Before the study of the nucleotide arrangement in the deoxy- 

 ribonucleic acid of rye germ could be undertaken, it was neces- 

 sary to examine the properties of deoxymethylcytidine 3',5'- 

 diphosphate whose production is indicative of the frequency of 

 this pyrimidine as a solitary nucleotides^. At the same time, the 

 two dinucleoside triphosphates comprising 5-methylcytidine and 

 either cytidine or thymidine were also studied. 



The investigation of the deoxyribonucleic acid of rye germ^^ 

 offered us, for the first time, the opportunity of comparing the 

 pyrimidine distribution in the total preparation with that found 

 in a complete series of seven fractions isolated through the frac- 

 tional dissociation of the histone nucleate. These fractions were, 

 in their distribution and composition, remarkably similar to those 

 previously isolated by the fractionation of the deoxyribonucleic 

 acid from wheat germ^^. The availability of such a complete 

 series of fractions permitted, moreover, a test of the validity of 

 the results given by the differential distribution analysis; a 

 weighted average of the values recorded for the individual frac- 

 tions could be compared with those given by the total nucleic 

 acid preparation that had not been subjected to fractionation and 

 showed an excellent agreement^^. 



The principal results of this study are, as regards the frequency 

 of soUtary pyrimidine nucleotides, shown schematically in Fig. 

 12. In this histogram, we give a brief description of the total 

 pyrimidine composition of all specimens examined — total deoxy- 

 ribonucleic acid and seven fractions — and depict the quantities 

 of soUtary nucleotide of each of the three pyrimidines as per cent 

 of the total constituent in the preparation. 5-Methylcytosine, it 

 will be seen, has a surprisingly high tendency, when compared 

 with the other 6-aminopyrimidine, cytosine, to occur as a solitary 

 unit, similar in this respect to thymine. I shall return to this point 

 later. 



e. Nucleotide arrangement in a microbial deoxyribonucleic 

 acid of the GC type 



Our survey has so far all been dealing with deoxyribonucleic 



