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CHAPTER 33 



OH 



OH 



or 



OH 



OH 



OH 



D-RIBOSE 



OH 



2'-DEOXY-D-RIBOSE 



FIGURE 33-5. Pentose sugars found in nucleic acids. 



formula of purine by having an NH2 group 

 in place of H at position 6, so that this com- 

 pound can be identified also as 6-amino- 

 purine. (A purine similar to adenine, having 

 a CH3 substitution on the NHo group at 

 position 6, has been found in limited amounts 

 in DNA, and is called, appropriately, 6- 

 methylaminopurine .) 



The other purine most frequent in DNA is 

 guanine (Figure 33-4). Guanine has an NH2 

 group at position 2 and an O in keto form at 

 position 6, so it can be called also 2-amino-6- 

 oxypurine. Note that differences among 

 purines lie largely in the groups attached at 

 the 2 and 6 positions of the double ring. 



D-ribose is a sugar (Figure 33-5a) contain- 

 ing five carbons, being, therefore, a pentose 

 sugar, of which four C are joined with an O 

 to form a five-membered ring. Figure 33-5a' 



employs the convention, used hereafter, of 

 not showing the carbons in the ring. DNA 

 contains a pentose sugar modified from the 

 D-ribose structure by the absence of an 

 oxygen at position 2', so that this sugar is 

 named 2'-deoxy-D-ribose, and is often called 

 2-deoxyribose, or deoxyribose (Figure 33-5b 

 and b'). 



Each organic N-containing purine or py- 

 rimidine base in DNA is normally joined to a 

 deoxyribose sugar to form the combination 

 called a deoxyriboside. The four main 

 deoxyribosides in DNA are those for cytosine, 

 called deoxycytidine, for thymine, called 

 (deoxy) thymidine, for adenine, called deoxya- 

 denosine, and for guanine, called deoxyguano- 

 sine. The structure for these is shown in 

 Figure 33-6. Note that the deoxyribose 

 sugar always joins to these bases at its V posi- 



