THE BIOCHEMISTRY OF PLANT VIRUSES 



53 



In addition to the above, many nucleic acids contain small amounts of 

 methylated purines and pyrimidines. So far these have not been detected in 

 virus nucleic acids. 5-Methylcytosine is not known to occur in viruses ( Wyatt, 

 1951), while 5-hydroxymethylcytosine is peculiar to certain bacteriophages 

 (Wyatt and Cohen, 1953). 



Certainly all four usual constituents of ribonucleic acid possess coenzyme 

 activity in one or more systems. 



A. The General Structure of Nucleic Acids 



The nucleic acids are divided into two main classes, the ribonucleic acids, 

 which contain D-ribofuranose, and the deoxyribonucleic acids, which contain 

 D-2-deoxyribofuranose. These sugars are in /8-iV-glycosidic linkage with 

 purines, which form 9-glycosides, and pyrimidines, which form 3-glycosides. 

 The bases commonly found in nucleic acids are shown in Table I. 



TABLE I 

 Bases Associated with Various Nucleio Acids 



Bases 



Ribonucleic acids 



DeoxjTibonucIeic acids 



Purines 

 Pyrimidines 



1. Nucleosides 



By suitable degradation methods the sugar glycosides of bases may be 

 liberated. These are called nucleosides, and have the general structure shown 

 in Formula II. 



BASE 



CH,OH 



\l 1/ 



I I 

 OH 



(11) 



The nucleosides are named for their bases. Thus, the adenine riboside is 

 called adenosine. The other nucleosides are guanosine, cytidine, uridine. 



