THE PROBLEM OF NUCLEOTIDE SEQUENCE IN DEOXYRIBONUCLEIC ACIDS 89 



TABLE VIII 



Examples of Pleromeric Relationships in Ribonucleic Acids 



Pleromers 



Source of . , 



RNA ' 1 ' 11V Normal pyrimidine 6-Am/6-K* Reference 



or natural satellite 

 (mole %) 



nucleotide (mole ",,) 



* Ratio of the molar sum of adenylic and cytidylic acids to that of guanylic, 

 uridylic and pseudo- or 5-fluorouridylic acids. 



t Ribothymidylic acid (11 mole %) included among the 6-keto nucleotides. 



germ nucleic acid [37] ; thymine and 5-bromouracil in the nucleic acid of 

 E. coli [42]. It goes without saying that, at the present state of our know- 

 ledge, such statements as the presence or absence of homotopy must be 

 taken as valid only in the most general statistical terms. Specific homotopic 

 replacements may, of course, have taken place even where no all-embracing 

 positional replacement can be affirmed. 



One could conclude provisionally that in the deoxyribonucleic acids 

 pleromers are not necessarily homotopes, and homotopes not necessarily 

 pleromers. Before the law of pleromerism all bases are equal as long as 

 they pair otT according to functional groups. But the rules of segregation 

 applied to them, once they have gone through the door, still are obscure. 

 These rules are, I am afraid, much less simple and predictable than would 

 please the advocates of biological automation. 



This brings me to the last topic of the present survey, namely, the 

 neighbour problem. It was, I believe, first stated in the following terms [10]. 

 "The frequency of methylcytosine being linked to guanine in a dinucleo- 

 tide as compared with that of cytosine being so attached is twenty times as 

 high in calf thymus nucleic acid, and thirteen times as high in the wheat 

 germ preparation, than would have been expected for non-selective in- 

 corporation. The unexpectedly high proportion of 5-methyldeoxycytidyIic 

 acid that is linked to deoxyguanylic acid [53, 54] makes one, in fact, 

 wonder whether it is not the adjoining nucleotide rather than the one 



