312 P. Alexander, S. F. Cousens and K. A. Stagey 



Table V 



Crosslinking of DNA in sperm heads 



Fraction of 

 ^11 7 J- ^ - J J ^c phosphate DI^ A rendered 



Alkylating agent used ^/^^^^ J^^^.^^^ insoluble in 



2M-NaCl * 



COOH • CH • CHg • CeH^ • N • (CH2CH2C1)2 ■) 

 NH2 CB. 3025 y 



COOH(CH2)3C6H4N(CH2CH2Cl)2 1 

 Chlorambucil J 



* Insolubility was defined as DNA removed by centrifuging at 20,000 g for 1 hour. If the 

 dispersion of the sperm heads is carried out carefully no DNA from untreated spenn heads is 

 removed under these conditions. 



occurs only between DNA molecules, and that reaction with 

 protein plays no part. 



Why are the nucleophilic alkylating agents 

 mutagenic? 



All attempts to obtain reaction between DNA and com- 

 pounds such as triacrylformal (see page 295) and thiol esters 

 (R-S-CO-R'), which react readily with amino groups in pro- 

 teins and single model substances, have in our hands been 

 quite unsuccessful. It seems likely that in native DNA the 

 purine and pyrimidines cannot readily undergo reactions for 

 steric or other reasons. On the basis of the Watson and Crick 

 model this seems very likely and the most vulnerable points 

 to attack in DNA are the phosphate groups. The reason why 

 the nucleophilic alkylating agents are so highly mutagenic 

 is that they are one of the few classes of substances which can 

 react with intact DNAf. Reaction with phosphate groups 

 may not be necessary per se for mutagenic action but it hap- 

 pens to be the only way by which DNA can be attacked easily 

 under mild conditions. 



t Alkylation of amino groups by epoxides probably occurs after the mole- 

 cule has been disrupted due to extensive reaction with the phosphate groups. 



