SUMMARIZING REMARKS 157 



mation is available about the relative frequencies of pTpBp and 

 pBpTp among the pyrimidine doublets or about the relative 

 abundance of adenine-bromouracil-adenine as compared with 

 that of adenine-thymine-adenine among the solitary pyrimidine 

 units. Until much more is known it would be safer to assume that 

 in the deoxyribonucleic acids pleromers are not necessarily 

 homotopes, and homotopes not necessarily pleromers. 



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

 the neighbor problem. It was, I believe, first stated in the following 

 terms^*^. 'The frequency of methylcytosine being linked to 

 guanine in a dinucleotide 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 incorporation. The 

 unexpectedly high proportion of 5-methyldeoxycytidylic acid that 

 is linked to deoxyguanylic acid^^'^^ makes one, in fact, wonder 

 whether it is not the adjoining nucleotide rather than the one op- 

 posite that directs the incorporation." This remarkable phenom- 

 enon — the high tendency of methylcytosine to be next to guanine 

 — was again touched upon in our study on the sequence charac- 

 teristics of the deoxyribonucleic acid of rye germ^*. Several 

 analogous observations also were discussed there of which one 

 may be mentioned. When, in this nucleic acid, the coupled 

 dipyrimidine units are surveyed, it is found that methylcytosine 

 has a greater tendency to be linked to either thymine or cytosine 

 than the latter has to associate with itself or with thymine; and 

 there is a definite bias in favor of methylcytosine-cytosine units. 



Somewhat similar observations have also been made in the 

 case of the ribonucleic acids in which the analysis of neighboring 

 tendencies is easier because of th.e facility of obtaining any given 

 nucleoside as either the 3'- or the 5'-phosphate. An examination 

 of this type has, for instance, been made with ribonucleic acid 

 preparations from different cellular portions of rat liver^^. A 

 particularly instructive example of the neighbor principle is of- 

 fered by the "soluble" ribonucleic acids of the cytoplasm con- 

 cerned with the specific transfer of amino acids^^, in which the 



References p. 159 



