Organization, Replication, and Types of DNA in Vivo 



307 



ADENINE THYMINE GUANINE CYSTOSINE 



Tuberculosis bacillus 15.1 



14.6 



34.9 



35.4 



Escherichia coli 



26.1 



23.9 



24.9 



25.1 



Vaccinia virus 



29.5 



29.9 



20.6 



20.3 



E. coli bacteriophage Tj 32.6 



32.6 



18.2 



16.6 



FIGURE 34-1. Base composition of DNA from various ori^unisms. 



purines always equals the total number of 

 DNA pyrimidines. While this regularity is 

 common to all the chromosomal DNA's 

 listed, there is nothing in the nature of the 

 primary structure of DNA which would help 

 explain it. However, the fact that the pri- 

 mary structure of DNA is the same in all 

 these organisms suggests that these regulari- 

 ties may be connected with some additional, 

 general, structural characteristic of chromo- 

 somal DNA. 



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 a long 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 hetero- 

 geneous in structure, no regularity is found 

 in the way in which the emergent beam is 

 refracted. But, if the material is composed 



of macromolecular units and/or molecular 

 subunits, which are spatially arranged in a 

 regular manner, then the emergent beam will 

 form what is called an X-ray diffraction 

 pattern. Moreover, a particular X-ray pattern 

 can be used to identify units and subunits 

 that are repeated at regular intervals of space. 

 Thus, it has been found that each nucleotide 

 in a DNA chain occupies a length of 3.4 A 

 along the chain, and that this repetition is 

 detectable by the characteristic X-ray diffrac- 

 tion pattern it produces. 



X-ray diffraction patterns have been ob- 

 tained for DNA from a variety of species. 

 In some cases the DNA was not removed 

 from the nucleus, in other cases it was re- 

 moved from the nucleus and separated from 

 protein as well. In all cases, provided the 

 DNA was suitably hydrated, essentially the 

 same patterns attributable to DNA were 

 found (see Figure 34-2). A study of these 



