Chemical Nature of Genes 



295 



Figure 33-2b' and Figure 33-2b" show suc- 

 cessive abbreviations of this formula, cor- 

 responding to those used for benzene. 



The N found in DNA is also found in a 

 derivative of the basic pyrimidine ring, called 

 a purine, which is composed of a pyrimidine 

 ring, minus the H atoms at positions 4 and 5, 

 to which is joined an imidazole ring (5- 

 membered), so that the carbons at these posi- 

 tions are shared by both rings (as shown in 



Figure 33-2c, and the shorthand forms in c' 

 and c")- Henceforth, the most abbreviated 

 structural representation will be used for 

 pyrimidines and purines. All pyriniidines 

 and purines act chemically as bases. 



Figure 33 3 includes various types of py- 

 rimidines, the names of those found in DNA 

 being underlined. Note that all the deriva- 

 tives of pyrimidine shown have an oxygen 

 added at position 2 to replace the H which is 



H 



I 



H-cf^>C-H 



I II 



k 



H 



h^Vh 



H-L^H 

 H 



a' 

 BENZENE 



N 



H 



I 



C-H 





H- 



H 



H 



1 



N 



PYRIMIDINE 



H-C: 



I 

 H 



sC— H 



c' 

 PURINE 



FIGURE 33-2. Relations/lip between certain ring compounds. 



