86 NUCLEIC ACIDS AND NUCLEOPROTEINS 



adenine, guanine, cytosine, and thymine in DNA, and the first 

 three and uracil in RNA. DNA appears somewhat more varied 

 in the array of its constituents than RNA; sateUites such as 5- 

 methylcytosine or 6-N-methyladenine are found occasionally, and 

 in the even coliphages 5-hydroxymethylcytosine occurs instead 

 of cytosine. What should be borne in mind is that, in general, 

 there are two purines and two pyrimidines and that one member 

 of each of these two groups carries, in its 6-position, an amino 

 group, the other a hydroxyl or, tautomerically, a keto group. 



4. DEOXYPENTOSE NUCLEIC ACIDS 



a. Composition and regularities 



As early as in 1949, our studies had led to the conclusion that 

 there existed a very large number of different deoxyribonucleic 

 acids: different as regards the proportions, and therefore the 

 sequence, of their component nucleotides, but of a composition 

 characteristic of the species, though not of the tissue, from which 

 they were derived^- ^' ^^. These conclusions have received ample 

 confirmation since that time^^-^^. In Table 20, I have selected a 

 few analyses from our own work^^- ^^"^^ in which DNA specimens 

 from various sources, easily seen to be different in composition, 

 are listed. The examples have been so chosen as to comprise 

 the entire spread in divergences found so far. The dissymmetry 

 ratio, i.e., the ratio of the sum of adenine + thymine to that of 

 guanine + cytosine + methylcytosine, ranges from approximate- 

 ly 1.9 to 0.4. We have referred^^ to deoxypentose nucleic acids 

 with a ratio above 1 as the AT type, to those with a ratio below 1 

 as the GC type; the latter appears to occur only in several micro- 

 organisms. Escherichia coli belongs to an intermediate class with 

 almost equal quantities of all components. 



It is, of course, not always possible to distinguish between 

 nucleic acids of very different origin through a study of their 

 total composition. Two examples of analytically indistinguishable 

 preparations^^' 20 are listed in Table 21. In such instances, it is 

 necessary to resort to other methods of differentiation; I shall 



