Chemical Nature of Genes 



299 



tion, the linkage being at position 3 of py- 

 rimidines and at position 9 in the case of 

 purines. 



In DNA, a phosphate group (PO4) is al- 

 ways joined to a deoxyriboside forming a 

 deoxyribonucleotide . The phosphate is at- 

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

 sugar, as shown in a generalized form in 

 Figure 33-7. This is shown specifically for 

 the deoxyribonucleotides containing the py- 

 rimidine cytosine and the purine adenine in 

 Figure 33-8. The deoxyriboside 5'-mono- 



phosphates of cytosine, thymine, adenine, and 

 guanine are called, respectively, deoxycytidylic 

 acid, thy mid y lie acid, deoxyadenylic acid, and 

 deoxyguanylic acid. In summary, then, the 

 basic unit of DNA is the deoxyribonucleotide 

 which is composed of a phosphate joined to 

 a deoxyriboside, which, in turn, is composed 

 of a deoxyribose sugar joined to an organic 

 base. These bases are either pyrimidines 

 (most commonly cytosine and thymine) or 

 purines (most commonly adenine and 

 guanine). 



FIGURE 33-6. Common deoxyribosides. 

 NH2 



OH H 



Deoxycytidine 



CH. 



OH H 



Thymidine 



PYRIMIDINE DEOXYRIBOSIDES 



OH H 



Deoxyadenosine 



OH H 



Deoxyguanosine 



PURINE DEOXYRIBOSIDES 



