The Chemical Basis of Heredity Determinants 331 



The Heterogeneity 



A chemist who intends to analyze a purified preparation of the 

 transforming principle will, of course, be concerned with the problem 

 as to whether the preparation represents a single chemical species or 

 a mixture of many. Both the biological and the chemical evidence 

 indicates that the latter may be the case. 



If one performs a transformation experiment using the DNA from 

 a donor carrying several transformable characters ("markers"), each 

 of the resulting transformed cells carries as a rule * only a single 

 marker. Doubly transformed cells occur as rarely as predicted from 

 the probability of two independent particles hitting the same cell. 

 Thus, the DNA of each cell seems to consist of many different mole- 

 cules, each of which determines a different hereditary character. 



Chemical evidence has been furnished by the important discovery 

 of DNA fractionation. It has been shown that DNA preparations 

 from calf thymus or from E. coli can be separated into several frac- 

 tions each differing in its proportions of individual purines and py- 

 rimidines. The compositions of whole DNA preparations recorded in 

 the literature are therefore not the compositions of individual molecules 

 but the averages of compositions. 



The differences in the proportions of purines and pyrimidines (or 

 their nucleotides) may be not the only manifestation of non-identity 

 of individual molecules; other differences may involve a difference in 

 sequence of nucleotides, in length of the molecule which is asymmetri- 

 cal, 7 or in some other unknown feature. 



The Molecule of the Transforming Principle 



If, as it seems, the molecules carrying different features are different, 

 it becomes important to estimate how many molecules of one kind are 

 necessary to transform one cell. Such an estimate has been made for 

 H. influenzae. This species is more convenient than pneumococcus 

 because the cultures of the latter contain DNAase, which tends to 

 obscure quantitative study. The total amount of DNA necessary to 

 transform one cell of H. influenzae was found to be 10~ 8 fig., which is 

 five times more than the total amount of DNA per cell (2 X 10 _9 fig.) 

 in this species. 21 If each molecule of DNA is different, then the 

 number of molecules of one kind necessary for transformation would 

 be of the order of five. If this number could be further reduced, 

 support would be gained for the hypothesis that practically all mole- 



* An interesting exception will be discussed on p. 332. 



