Chemical Nature of Genes 



257 



OH 



OH 



OH 



or 



OH 



D-RIBOSE 



or 



OH 



2-DEOXY-D-RIBOSE 



figure 19-5. Pentoses found in nucleic acids. 



called 2-amino-6-oxypurine. The purines 

 differ largely in the groups attached at the 

 2 and 6 positions of the double ring. 



Pentoses. D-ribose is a sugar (Figure 

 19-5 a) containing five carbons (being, 

 therefore, a pentose), four of which are 

 joined with an O to form a five-membered 

 ring. Figure 19-5 a' employs the conven- 

 tion, used hereafter, of not showing the car- 

 bons of the ring. The carbons in pentose 

 are given primed numbers to indicate their 

 positions. DNA contains a pentose 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 simply, deoxyribose 

 (Figures 19-5b and b')- 



Deoxyribosides. Each purine or pyrimi- 



dine base in DNA is joined to a deoxyribose 

 sugar to form the combination called a de- 

 oxyribonucleoside or deoxy riboside. The 

 four main deoxyribosides in DNA are: de- 

 oxy cytidine (for cytosine); (deoxy) thy- 

 midine (for thymine); deoxy adenosine (for 

 adenine); and deoxyguanosine (for gua- 

 nine). The structure for these is shown in 

 Figure 19-6. Note that the deoxyribose 

 always joins to the organic base at its Y 

 position. The linkage involved occurs at 

 position 3 in pyrimidines and at position 9 

 in purines. 



Deoxyribotides. In DNA a phosphate 

 (PO^ group is always joined to a deoxy- 

 riboside, forming a deoxyribonucleotide or 

 deoxyribotide. The phosphate is attached 

 either at the 3' or 5' position of the sugar 



