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CHAPTER 19 



figure 19-10. Terminology for nucleic acids and their components. 



7. DNA is a linear, unbranched, polymer — 

 a reasonable finding if one expected DNA 

 to represent a sequence of genes. Just 

 as interstitial genes are bipolar (see p. 

 1 89 ) , so are interstitial segments of DNA, 

 since each deoxyriboside joins only to 

 two other deoxyribosides via its 3' and 5' 

 sugar linkages to phosphate. 



In its cellular location and in all of the re- 

 spects mentioned above, the observations 

 are consistent with the view that DNA either 

 is, or is intimately associated with, the ge- 

 netic material. 



Chemical Composition of RNA 



In addition to DNA another type of nucleic- 

 acid is found in the chromosome. This is 

 ribonucleic acid or RNA. Normally chro- 

 mosomal RNA is found in combination with 

 protein in the form of rihonucleoprotein. 



Because the RNA content of chromosomes 

 varies within a cell and among diploid cells 

 of the same organism according to meta- 

 bolic activity, RNA is unlikely to be the 

 chemical basis of genes in typical (DNA- 

 containing) chromosomes. Nevertheless, 

 let us discuss the chemical composition of 

 RNA, noting in particular its resemblance 

 to DNA. 



Chromosomal RNA, like DNA, is a long, 

 unbranched polymer with the basic unit being 

 a ribonucleotide or ribotide. The ribotide 

 is like the deoxyribotide, for it, too, is a 

 combination of organic base plus pentose 

 plus phosphate; one way in which it differs 

 is that the pentose is D-ribose (Figure 19-5) 

 rather than 2'-deoxy-D-ribose. Another dif- 

 ference is found in RNA's pyrimidines. The 

 two pyrimidines commonly found in RNA 

 are cytosine (also common in DNA) and 

 uracil (2,6-oxypyrimidine — -not found in 



