266 



( II A I'll R 20 



ADENINE THYMINE GUANINE CYTOSINE 



E. coli bacteriophage T 2 32.6 



32.6 



18.2 



16.6 



figure 20-2. Base composition of DNA from various organisms. (*5-/;v- 

 droxymethyl cytosine. ) 



tissues in the same and among different 



human beings. Nevertheless, a genome is 



expected to contain many DNA molecules 



which differ in base content and sequence. 



A _l_ t 



The variation found in — — in differ- 



G + C 



ent species — the ratio is about 0.4 for the 



tubercle bacillus and about 1 .8 for the sea 



urchin — is consistent with our chemical 



knowledge, since the DNA strand imposes 



no limitation upon either the types or the 



frequencies of the bases present along the 



length of the fiber. However, the amount 



of A and the amount of T in the DNA of 



a given species are remarkably equal as are 



the amounts of G and C (Figure 20-2). 



Since in each species A = T and G = C, 



it is also true that A + G = T + C;in other 



words, the total number of DNA purines 



always equals the total number of DNA 

 pyrimidines. Although this regularity is 

 common to all the chromosomal DNA's 

 listed, there is nothing in the primary struc- 

 ture of DNA which helps to explain this 

 significant fact. That the primary structure 

 of DNA is the same in all these organisms 

 suggests, however, that these regularities 

 may be connected with some additional, 

 general characteristic of chromosomal DNA 

 structure. 



An understanding of the basis for the 

 A = T and G = C relationships may come 

 from studies of an entirely different kind. 

 It has been known for some time that a 

 beam of X rays is bent or refracted when it 

 passes through material. If the material 

 through which the rays pass is completely 

 heterogeneous in structure and orientation. 



