details). In some viruses, for example the tobacco mosaic virus, RNA 

 apparently also plays a major genetic role (Figure 2-4). 



The molecular organization of nucleic acids, especially DNA, has 

 received a good deal of attention in recent years. In view of the fact 

 that DNA appears to represent the basis of organization of genetic in- 

 formation, the question of structure is vital. The classic notion, primarily 

 that of P. A. Levene, was that the nucleic acid molecule is made up of 



m-'mnims-aw- - 



Figure 2-4. Electron Micrograph of Tobacco Mosaic Virus Protein Rods. 

 Approximately 70,000x. (From WyckofY, R. W. G., 1958. "The World of 

 the Electron Microscope," Yale University Press, New Haven. Conn., Fig. a, 

 Plate X. Courtesy of Dr. R. W. G. Wyckoff, University of Arizona.) 



repeating units of the four bases in equal amounts. This has been re- 

 ferred to as the tetranucleotide hypothesis. One expectation from this 

 hypothesis would be that the four bases should show a 1:1:1:1 corre- 

 spondence. Biochemical work over the past twenty years has shown 

 quite clearly that this expectation is not realized (Davidson, 1957). 

 Most analyses of DNA show that there is a 1:1 relationship between 

 adenine and thymine and between guanine and cytosine but the sum of 

 the last two may be greater or less than the sum of the first two. This 

 fact alone suggests intimate association of specific pairs of bases. From 

 the biochemical data and from X-ray diffraction studies, Watson and 



18 / CHAPTER 2 



