Chemical Nature of Genes 



259 



(a generalized form appears in Figure 19-7 ). 

 This combination is shown specifically for 

 the deoxyribotides containing the pyrimidine 

 cytosine and the purine adenine in Figure 

 19-8. The deoxyriboside 5'-monophos- 

 phates of cytosine. thymine, adenine, and 

 guanine are called, respectively, deoxycyti- 

 dylic acid, thymidylic acid, deoxyadenylic 

 acid, and deoxyguanylic acid. In summary, 

 then, the basic unit of DNA is the deoxy- 



ribotide which is composed of a phosphate 

 joined to a deoxyriboside; this, in turn, is 

 composed of a deoxyribose joined to an or- 

 ganic base. These bases are either pyrimi- 

 dines (most commonly cytosine and thy- 

 mine) or purines (most commonly adenine 

 and guanine). 



Poly deoxyribotides. Chromosomal DNA 

 occurs not as single deoxyribotides but as 

 polydeoxyribonucleotides or polydeoxyribo- 



NHo 



Deoxycytidine 3 -monophosphate 



OH H 



Deoxycytidine 5'-monophosphate 



or 



Deoxycytidylic acid 



NH. 



NH 



Deoxyadenosine 3 -monophosphate 



figure 19-8. Specific deoxyribotides 



OH 



Deoxyadenosine 5 -monophosphate 



or 



Deoxyadenylic acid 



