SUMMARIZING REMARKS 155 



Pleromeric relationships in ribonucleic acids are much more 

 difficult to estabhsh, as the only complementariness* useful for 

 scrutiny is the equality of the 6-amino and 6-keto nucleotides^^. 

 Of the three examples listed in Table 36 only the one deahng 

 with the incorporation of 5-fluorouraciP^ carries conviction. It 

 is not impossible that, when more careful analyses become 

 available, 8-azaguanine and guanine will also emerge as plero- 

 mers. 



The usefulness of the concept of homotopy is, for the time 

 being, more evident with respect to the amino acid sequence in 

 proteins than to the nucleotide arrangement in nucleic acids. 

 More and more instances of such homotopic relationships are 

 being revealed as knowledge on specific sequences in proteins 

 increases. As concerns the deoxyribonucleic acids, we are largely 

 limited to the consideration of simple sequences, such as purine- 

 pyrimidine-purine, or of small bunches of pyrimidine nucleotides. 

 The development of the study of larger clusters, mentioned 

 before, will doubtless contribute much to our knowledge. I have 

 emphasized above a possible instance of homotopy, namely, the 

 remarkable similarity in distribution, as solitary nucleotides, of 

 the two 5-methylpyrimidines (thymine and 5-methylcytosine) in 

 rye germ deoxyribonucleic acid^^. 



At the present stage of our knowledge statements on the 

 presence or absence of homotopy can be valid only in the most 

 general statistical terms. Specific homotopic replacements may, 

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

 tional replacement can be affirmed. As concerns the replacement 

 of thymine by a foreign analogue, discussed above in more detail, 

 it ought to be understood that the observation of an ostensibly 

 random replacement of thymine by 5-bromouracil does not ex- 

 clude the possibility that the analogue may be involved in certain 

 non-random preferences for neighbors. For instance, no infor- 



* This regularity — in the absence of the other pairing regularities 

 characteristic of deoxyribonucleic acids — may, formally, be taken to mean 

 that in ribonucleic acids adenine can pair not only with uracil but also 

 with guanine, and cytosine not only with guanine but also with uracil. 



References p. 159 



