2.")4 



CHAPTER 19 



Figure 19—3 shows the structural formulae 

 for various types of pyrimidine. The names 

 underlined are found in DNA. All the dc- 

 rivatives shown o\' the basic pyrimidine ring 

 have an oxygen at position 2 replacing the 

 H which is now at position 3. This oxygen 



R 

 is shown in the keto form (O = C<^ , with 



R 



R representing an atom or group other than 



H). Two pyrimidines are commonly found 

 in DNA: cytosine and thymine. Cytosine 

 differs from the basic pyrimidine ring by 

 having an amino (NH L .) group attached to 

 the C at position 6 instead of the H. Conse- 

 quently, cytosine can also be called 6-amino- 

 2-oxypyrimidine. Replacement of the H at 

 position 5 in cytosine by a methyl (CH. { ) 

 group produces 5-methyl cytosine; this DNA 

 pyrimidine is found in appreciable amounts 



H 



•is 



H— Ci sC— H 



I II 



H— Q , 4C— H 



H 



a 



a' 

 BENZENE 



H 



Ni sC— H 



I II 



H— Q , 4C— H 



b' 

 PYRIMIDINE 



b" 



H 



Ni C 

 H— O C 



H 



*C— H 



H 



PURINE 



[KiLRi 19-2. Relationship between certain ring compounds. 



