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CHAPTER 33 



sides (deoxyribo- and ribosides, respectively), 

 composed of a pentose sugar (2'-deoxy-D- 

 ribose and D-ribose, respectively), joined to 

 a pyrimidine (usually cytosine or thymine and 

 cytosine or uracil, respectively) or to a purine 

 (usually adenine or guanine). Some of this 

 terminology is summarized in tabular form 

 in Figure 33-10. 



Ahhough the RNA in chromosomes does 

 not possess either the proper quantitative 

 variation or constancy we would expect of 

 ordinary chromosomal genes, it does possess 

 the same linear organization as DNA, as 

 would be expected for linearly arranged re- 

 cons. Suffice it to say at this point that 

 certain viruses (influenza, poliomyelitis and 

 other encephalitic viruses, viruses like tobacco 

 mosaic virus which attack plants, and even 

 a virus attacking bacteria)' possess genetic 

 properties, but do not contain DNA. These 

 viruses are composed primarily of ribo- 

 1 See T. Loeb and N. D. Zinder (1961). 



nucleoprotein. Since DNA rather than pro- 

 tein is favored as being the genetic chemical 

 under typical chromosomal conditions, it is 

 reasonable to entertain the view that it is 

 RNA rather than the protein which is the 

 chemical basis of genetic specification in these 

 particular viruses. 



What we will attempt to do, in subsequent 

 Chapters, is to present additional evidence 

 that tests the view that DNA typically (and 

 RNA in special cases) either is the genetic 

 material or is intimately associated with it. 

 Clearly we are seeking ultimately to deter- 

 mine the chemical units of the genetic ma- 

 terial which may correspond to the cistron 

 and the recon, and the chemical basis for the 

 mutation of single genes. In view of the like- 

 lihood that the cistron contains more than a 

 single recon, it should be realized that the 

 chemical units, which correspond to the 

 cistron and recon, are expected to be diff'erent, 

 at least quantitatively. 



FIGURE 33-10. Terminology for nucleic acids and their components. 



