ISOLATION AND COMPOSITION OF DEOXYPENTOSE NUCLEIC ACIDS 



365 



TABLE XVI 



S-Methylcytosine Distribution in Fractions of Calf Thymus 

 Sodium Deoxyribonucleate* 



Fraction 

 1 2 3 



NaCl molarity in extraction of nucleohistone gel 



Corrected mean ■proportions 

 Thymine 

 Cytosine 

 5-Methylcytosine 



Molar ratio 



Thymine to 5-methylcytosine 

 Cytosine to 5-methylcytosine 



0.40 0.45 1.7 



2.6 



• The mean proportions of each constituent have been corrected on the assumption that one-half of the 

 nucleic acid P is contributed by the pyrimidine nucleotide. In one hydrolysis experiment with Fraction 4 a 

 minute amount of 5-methylcytosine (about 0.6 mole) was found. 



pyrimidine in the several fractions were found. This finding, it may be 

 pointed out, is difficult to reconcile with the structural hypothesis of Wat- 

 son and Crick-*^ (compare Chapter 13) which would have led to the expecta- 

 tion of a random replacement of cytosine by the methyl derivative. (See 

 also Section X.2.) 



It is too early to attempt a detailed consideration of the reasons under- 

 lying the fractionation procedure. That nucleic acid chains relatively rich 

 in guanylic and cytidylic acids are detached more easily than those in which 

 adenylic and thymidylic acids predominate indicates that some property 

 inherent in their composition or sequence reduces the strength with which 

 they are bound to the histone. Brown and Watson^*^ have, on the basis of 

 Cavalieri's work,'^^ suggested that hydrogen-bonding between the 2-amino 

 group of guanine and a phosphate group of the sugar-phosphate backbone 

 of the nucleic acid could reduce the acidic properties of structures in which 

 this type of interaction occurs frequently and thereby weaken the link be- 

 tween protein and nucleic acid. 



The fractionation experiments, which are discussed in detail in the pub- 

 lications cited before,^^*"-*' suggest that the deoxypentose nucleic acid of a 

 given cell is composed of a very large number of differently constituted 

 individuals and that it is possible to achieve the resolution of this entire 

 spectrum of structural gradations (descending contents of guanine and 



"2 J. D. Watson and F. H. C. Crick, Nature 171, 737, 964 (1953) ; Cold Spring Harbor 

 Symposia Quant. Biol. 18, 123 (1953). 



