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CHAPTER 21 



CH; 



Thymine 



Uracil 



5-Bromo uracil 



NH 



NH, 



CH; 



CT N' 

 H 



5-Methyl cytosine 



NH. 



^HN 



NH^ NT N 

 H 

 Guanine 



HN 



Hypoxanthine 



figure 21-4. Various bases utilized 

 in in vitro synthesis of DNA. Ar- 

 rows point to groups capable of 

 typical H-bonding. 



oxyuridinc 5'-triphosphate (or 5-bromo 

 or 5-fluoro deoxyuridine 5'-triphosphate), 

 </CPPP, </APPP, and </GPPP, DNA syn- 

 thesis occurs. Similarly, 5-mcthyl cytosine 

 or 5-bromo or 5-fluoro cytosine can substi- 

 tute for cytosine. On the other hand, the 

 substitution of hypoxanthine for guanine 

 does not support DNA synthesis as well as 

 do the other substitutions mentioned. This 



result is expected from the hypothesis under 

 test, since the former has one less H-bonding 

 site than the latter. Moreover — as pre- 

 dicted — uracil, 5-bromo uracil, and 5-fluoro 

 uracil will not substitute for C, A, or G in 

 dCPPP, <7APPP, or JGPPP, respectively. 

 Similarly, 5-mcthyl, 5-bromo, and 5-fluoro 

 cytosine replace cytosine specifically; hypo- 

 xanthine replaces only guanine. Although hy- 



