370 



ERWIN CHARGAFF 



TABLE XVII 

 MoLAK Relationships in All Deoxypentose Nucleic Acids* 



* The mean ratios and their standard errors are given for each table. In Table VII, Nos. 1, 2, and 11 were 

 not included; in VIII, No. 1; in IX, Nos. 11, 15, and 16; in XII, Nos. 1, 2, and 15. The values for cytosine 

 refer also to 5-methylcyto8ine or 6-hydroxymethylcytosine. 



adenine to thymine equals 1. (c) The molar ratio of guanine to cytosine 

 equals 1. (d) The number of 6-amino groups is the same as that of 6-keto 

 groups."^ It is, in fact, because of these relationships that it is possible to 

 state that, for instance, in the deoxypentose nucleic acids of wheat germ 

 5-methylcytosine substitues for part of the cytosine,^" that in certain phage 

 nucleic acids 5-hydroxymethylcytosine replaces cytosine entirely,^*' or that, 

 as has been shown recently ,^^^ 5-bromo- or 5-iodouracil may under circum- 

 stances take the place of thymine. It is not unlikely that more instances of 

 such substitution will be discovered. What appears remarkable, however, 

 is that up to this time they have been found only among the pyrimidines 

 and that no purine satellite has been revealed. 



Some of these relationships are, with respect to the composition of the 

 nucleic acids of different mammalian genera (Table VI), illustrated in Fig. 

 7. A complete survey of almost all data assembled in Tables V-XIII is 

 provided in Table XVII. The above-mentioned ratios were calculated 

 separately for each entry and their average values and standard errors com- 

 puted for each tabular division. (The assistance of Dr. D. Elson in these 

 calculations is gratefully acknowledged.) It will be seen that the agreement 



"1 This means that the sum of guanine and thymine equals that of adenine and 

 cytosine (+ methylcytosine). Originally, the ratio of all amino groups to enolic 

 hydroxyls was introduced.'" The present expression is preferable, especially in 

 the light of recent findings on pentose nucleic acids.*" 



"2 D. Elson and E. Chargaff, Nature 173, 1037 (1954). 



