15:3/ X-ray Analyses of Proteins and Nucleic Acids 



279 



(where the corners without letters in the ring are to be interpreted as 

 carbon atoms). The two pyrimidines found in DNA are cytosine (C) 1 

 and thymine (T), represented structurally by 



NH 2 



cytosine 



H 



H 



and 



HO 



thymine 



In RNA, two pyrimidines are also found. These are cytosine and uracil. 

 The latter has the structure 



OH 



nAh 



HO^ X N^ 



uracil 



In both RNA and DNA, the same two purine-derived bases occur. 

 These are called adenine (A) and guanine (G). 2 The structual formulas of 

 adenine and guanine are 



NH 



NH 



adenine 



guanine 



As mentioned above, most genetic information in plant and animal 



1 In certain bacteriophages, such as the coliform phages T2 and T4, the cytosine 

 in the DNA is replaced by 5-hydroxy-methyl cytosine. 



2 Adenine-ribose-phosphate is the nucleotide AMP which was introduced in 

 Chapter 8. It can be condensed with additional phosphate groups to form the 

 energy-transport compounds, ADP and ATP. 



