89 



SIDE VIEW 



I 



Figure V-3.— DNA (deoxyribonucleic acid) consists of two polymers linked 

 together by pairs of purine and pyrimidine bases. Of the four types of bases, 

 adenine can pair with thymine and guanine can pair with cytosine (Watson-Crick 

 pairing rules: A -T; G-C). 



