296 



CHAPTER 33 



NH2 



NH 



CYTOSINE 

 (6-amino-2-oxypyrimidine) 



NHo 



5-METHYL CYTOSINE 

 (6-amino-2-oxy-5- 

 methylpyrimidine) 



CH2OH 



5-HYDROXYMETHYL CYTOSINE 



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



methylpyrimidine) 



URACIL 

 (2,6-oxypyrimldine) 



THYMINE 

 (2,6-oxy-5-methylpyrimidine) 



(5-methyl uracil) 



FIGURE 33-3. Pyrimidines. 



Names of pyrimidines found 



in DNA are underlined. 



relocated at position 3. This O is shown in 



the keto form (O = C^ , where R repre- 

 \R 



sents an atom or group other than H). One 

 of the two pyrimidine derivatives most com- 

 monly found in DNA is cytosine. Cytosine 

 differs from pyrimidine by having also an 

 amino group (NH2) substituted for the H 

 attached to the C at position 6. Accordingly, 

 cytosine can also be called 6-amino-2-oxypy- 

 rimidine. (Substitution, in cytosine, of CH3 

 for the H attached at position 5 produces 

 5-methyl cytosine, a DNA pyrimidine found in 

 appreciable amounts in wheat germ, and in 

 trace amounts in mammals, fish, and insects. 

 Another pyrimidine, found only in the DNA 

 of certain viruses that attack bacteria, has a 



hydroxymethyl group (CH2OH) replacing the 

 H at position 5 of cytosine, and is therefore 

 called 5-hydroxymethyl cytosine.) 



The other most frequently occurring py- 

 rimidine in DNA is thymine. Thymine is 

 unique in having a keto group replacing the 

 H attached to the C at position 6, and has 

 also replaced the H at position 5 by a methyl 

 group. So thymine can be called 2,6-oxy- 

 5-methylpyrimidine. Note that the differ- 

 ences between pyrimidines lie primarily in the 

 variation in the groups present at the 5 and 6 

 positions in the ring. 



Figure 33 4 shows the structural formulae 

 for various purines, the names of those found 

 in DNA being underlined. One of the two 

 purines which commonly occur in DNA is 

 adenine. Adenine differs from the basic 



