Chemical Nature of Genes 



255 



Pyrimidine I 5 



N 



NH, 



(6-amino-2-oxy pyrimidine) 



NH, 



NH. 



CH, 



CH 2 OH 



5-METHYL CYTOSINE 

 (6-amino-2-oxy-5- 

 methylpyrimidine) 



5-HYDROXYMETHYL CYTOSINE 



(6-amino-2-oxy-5-hydroxy- 



methyl pyrimidine) 



figure 19-3. Pyrimi- 



dines. Names of pyrimi- 



dines occurring in DNA 



are underlined. 



in wheat germ and in trace amounts in mam- 

 mals, fish, and insects. Another pyrimidine, 

 found only in the DNA of certain viruses 

 attacking bacteria, has a hydroxymethyl 

 (CH.OH) group replacing the H at position 

 5 of cytosine. and is therefore called 5 -hy- 

 droxy methyl cytosine. 



The other pyrimidine commonly found in 

 DNA is thymine. Thymine is unique in 

 having a keto group replace the H attached 

 to the C at position 6; in addition a methyl 

 group replaces the H at position 5. So 

 thymine can also be called 2,6-oxy-5-methyl- 

 pyrimidine. Note that all the pyrimidines 

 shown differ primarily in the groups present 

 at the 5 and 6 positions in the ring. 



Figure 19-4 shows the structural formulae 

 for various purines; those found in DNA are 

 underlined. Two purines are commonly 

 found in DNA: adenine and guanine. Ade- 

 nine differs from the basic formula of purine 

 by having an NH. group in place of H at 

 position 6; therefore this compound can also 

 be identified as 6-amino-purine. A deriva- 

 tive of adenine has a CH ;! substitute for an 

 H in the NH 2 group at position 6 with 6- 

 methylaminopurine resulting; limited amounts 

 of this purine have been found in DNA. 



The other purine most frequent in DNA 

 is guanine (Figure 19-4). Since guanine 

 has an NH 2 group at position 2 and an O 

 in keto form at position 6, it can also be 



