Pyrimidine Moieties in Animals, Plants, and Bacteria 



11 



(RNA) and the translation of the Informational-RNA for specific 

 protein synthesis. 



The key to the understanding of tliese concepts is tlie model for 

 the structure of DNA proposed in 1953 by Watson and Crick (77). 

 Watson and Crick suggested that DNA consists of two helical 

 polynucleotide chains of opposite polarity which are twined round 

 one another (Fig. 2). Each chain is built from four mononucleo- 

 tide units which are joined together by 3', 5' phosphodiester 

 bonds. Each of the four mononucleotide units consists of either a 



Fig. 2. Simplified model of die DNA double helix showing hydrogen bonding 



and the DNA strands of opposite polarity. P = orthophosphate; S = deoxyribose; 



A = adenine; T = thymine; G = guanine; G = cytosine. 



