132 NUCLEOTIDE SEQUENCE IN DEOXYRIBONUCLEIC ACIDS 



equals that of the sum of cytosine and 5-methylcytosine, I would 

 say that these two 6-amino pyrimidines are pleromeric. 



c. Solitary and bunched nucleotides 



A pyrimidine nucleotide that, within the polynucleotide chain of 

 a nucleic acid, is contiguous only to purine nucleotides, will be 

 referred to as "solitary"; pyrimidine nucleotides that occur in 



A A A A A A 'A A 'y\ 





/ 



/ 



/ 



/ 



VV.\/V.VVV. 



/ 



■■& 



A / 



/ 



V 



/ 



V 



V. 



Fig. 11. Schematic representation of a fragment of a double strand of a 



deoxyribonucleic acid. The purines (A, adenine; G, guanine) are depicted 



in black, the pyrimidines (C, cytosine; T, thymine) in white. 



tracts of two or more, flanked on both sides by purine nucleotides, 

 will be designated as "bunched''^^. In the diagram shown in Fig. 

 11, soUtary thymidylic acid, for instance, appears in positions 3, 9 

 and 8', bunched pyrimidine nucleotides are seen in positions 5-7. 



4. EARLY ATTEMPTS AT SEQUENCE INVESTIGATION 



Since 1947, when in an early review on deoxyribonucleic acids 

 (p. 32 of Ref. 22), I first discussed the possible importance of 

 variations in the nucleotide sequence for the biological specificity 

 of a deoxyribonucleic acid, this problem has remained among the 



