THE CHEMICAL BASIS OF HEREDITY 



TABLE 1.2 

 Bas« Ratios of some DNA's and RNA's 



DNA 



RNA 



Certain conclusions can be drawn from these findings. In the first 

 place, there is a constancy in base composition, such that repeated 

 analyses of nucleic acids from particular organisms always give the same 

 results. Secondly, the average base composition of the nucleic acids is 

 species specific, differing from one organism to another. Thirdly, there 

 seems to be more species variation in the DNA ratios than in the RNA 

 ratios, but, since these values are averages of large numbers of different 

 molecules, more than one interpretation is possible. These findings pro- 

 vide no evidence for or against the view that the total DNA and RNA 

 extracted from the cell consists of a mixture of molecules, very hetero- 

 geneous in nucleotide sequence. As yet, most of the evidence in support 

 of this view comes from genetic, not from chemical, experiments. We 

 shall return to this question later. 



Crystallographers have studied the three-dimensional configurations of 

 DNA and of RNA molecules by means of X-ray diffraction, a method 

 which reveals the regularities in atomic arrangement. The DNA prepara- 

 tions from many different organisms all show the same characteristic 

 repeating pattern, which is best interpreted as resulting from the organ- 



