DEOXYPENTOSE NUCLEIC ACIDS 55 



the availability of uracil is amply demonstrated by the absence 

 of uracil from deoxypentose nucleic acids. 



As regards the 5-substituted cytosine derivatives, the situation 

 is even more disturbing. One must surely assume that both 

 cytosine and 5-methylcytosine are available to the cell during the 

 formation of certain deoxypentose nucleic acids, since both 

 pyrimidines are incorporated in what appears to be constant 

 proportions. For instance, three different groups of workers, 

 analyzing several independently prepared specimens of the deoxy- 

 pentose nucleic acid of wheat germ, found the following figures 

 for the 5-methylcytosine content (as moles per 100 gram-atoms 

 of phosphorus)^^'^^'^*: 5.8, 6.0, 5.7. Similarly, the corresponding 

 proportion of 5-methylcytosine in the nucleates from bovine tis- 

 sues is around 1.3 (see the survey in Ref. 1). This constancy of 

 incorporation must be due either to a specific selection operating 

 during the reproduction of the nucleic acids or, much less likely, 

 to an unexpectedly steady level in the supply of these pyrimidines 

 or their precursors in the cell. In the latter case, the degree of 

 incorporation of a pyrimidine and its 5-substituted analogue 

 would be governed by the laws of probability and there could be 

 no preferential accumulation of the satellite in certain portions 

 of the polynucleotide chain or in one or the other of the fractions 

 into which a total deoxypentose nucleic acid preparation can be 

 separated. But the available evidence suggests that 5-methyl- 

 cytosine does not replace cytosine at random and that the scheme 

 is either incorrect or incomplete in some essential features. This 

 evidence is twofold. 



(1) During the fractionation of calf thymus deoxyribonucleic 

 acid, which will be mentioned in the concluding section of this 

 article, very significant divergences in the concentration of 5- 

 methylcytosine in the several fractions were found (compare 

 Table 17 which is taken from Ref. 5). Similar results, soon to 

 be published, have been obtained with fractions of the deoxy- 

 pentose nucleic acid of wheat germ^^. 



(2) The action of pancreatic deoxyribonuclease on deoxy-^ 

 ribonucleic acids results in the production of a considerable; 



References p. 60 



