CHAPTER IX 



NUCLEOTIDES AND NUCLEIC ACIDS 



It requires but a short acquaintance with biochemistry to 

 reahze that the importance of nucleotides and nucleic acids 

 is comparable with that of amino-acids and proteins. 

 A nucleotide consists of a heterocyclic nitrogen compound, 

 such as nicotinamide, a purine or a pyrimidine, joined to a 

 sugar to which one or more orthophosphate groups are 

 attached. The sugar is either a pentose (e.g. ribose) or a 

 deoxypentose, and the dephosphorylated form of a nucleo- 

 tide is known as a nucleoside. (The latter is really a N- 

 glycoside and a nucleotide is therefore a phosphorylated 

 N-glycoside.) Natural pyrimidines and purines include 



cytosine (4-amino-2-ketopyrimidine), uracil (2:4-diketo- 

 pyrimidine), thymine (5-methyluracil), adenine (4-amino- 

 purine) and guanine (2-amino-4-ketopurine).^ The corre- 

 sponding ribose nucleosides are known as cytidine, uridine, 

 adenosine and guanosine, and the nucleotides as cytidylic, 

 uridylic, adenylic and guanylic acid respectively. Plants and 

 animals also contain 5-methylcytosine, but contrary to 

 earlier reports, this pyrimidine is apparently not present in 

 micro-organisms [51, 57]. 



Nucleotides are constituents of nucleic acids and also of 

 the prosthetic groups and co-factors of many enzyme sys- 

 tems concerned with transfer reactions (e.g. ATP in energy 

 transfer; DPN, TPN, FMN and FADN in hydrogen transfer; 



^ New system of numbering recommended by the Chemical 

 Society {J. Chern. Soc, 5064 (1952) )• 



126 



