16 I The Process of Evolution 



Fig. 1.4 I The structure of DNA. Upper left, complementary chains 

 of nucleotides. Letters, bases; five-sided figures, pentose sugars; circles, 

 phosphate groups; double lines, hydrogen bonds. Lower left, detail of 

 segment of one strand. Right, model of double helix in which the 

 nucleotide chains are arranged. Ribbons, phosphate-sugar backbone; 

 bars, paired bases joining backbones. This double-helix configuration is 

 known as the Watson-Crick model. (After Sinnott, Dun7i, and Dob- 

 zhansky, 1958, Principles of Genetics, McGraw-Hill.) 



A^^T 



C=G 



T = A 



G = C 



Base 

 (e.g., thymine) 



Base 

 (e.g., adenine) 



Base 



(e.g., guanine) 



- Sugar 



- Sugar 



- Sugar 



Base - Sugar 

 (e.g., cytosine) 



Phosphate 



Phosphate 



Phosphate 



