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ERNST FREESE 



leplication. Most agents have been tet^ted only on cells or organisms 

 and it is not clear whether they affect DNA directly or only indirectly, 

 e.g., by alteration of a nucleic acid jirecursor which thus becomes 

 mutagenic. 



A. IMIIBITION OF NUCLEIC ACID PRFX'URSORS 



There are many agents that interfere with the normal synthesis of 

 nucleic acid precursors (for review see Handschumacher and Welch, 

 1960) ; in addition, a number of mutants have been isolated that are 

 blocked in one of the pathways toward nucleic acids. Some of the agents 

 or mutants interfere with the synthesis of purines, others of pyrimidines, 

 and again others are more specific and affect only the synthesis of one 

 base, e.g., thymine, adenine, or guanine, sometimes only at the nucleo- 

 tide level. Some of the inhibitors of the nucleic acid pathways that have 

 shown a significant mutagenic effect are smnmarized in Table I. The 



Mutagenic Agents and 



TABLE! 

 Mutations Which Interfere with the Normal Formation of Nucleic Acid Precursors 



