Mutagenic Alkylating Agents 



311 



Table IV 



The initial effect of alkylation on the size of DNA 



effect of internal crosslinking, and the difference in appear- 

 ance between untreated * DNA and DNA after reaction with 

 a nitrogen mustard is most marked. On drying, the various 

 strands in the internally crosslinked DNA molecule are pulled 

 together to give the patterns seen. In solution, the electro- 

 static repulsion of the phosphate groups opens up the molecule 

 into a more sponge-like structure. 



Intermolecular crosslinking of DNA is found after treat- 

 ment of the sperm heads with bifunctional alkylating agents. 

 The untreated nucleoprotamine dissolves completely in 2 m 

 sodium chloride to give highly viscous solutions but after 

 reaction with bifunctional alkylating agents a proportion of 

 the nucleoprotamine is present as a highly swollen gel, the 

 presence of which cannot usually be detected by inspection. 

 Centrifuging for one hour at 20,000 g spins down this gel 

 without producing any sediment in the control solution. The 

 proportion of DNA present as gel after a variety of treatments 

 is shown in Table V. The gel is composed entirely of DNA and 

 contains no protamine. This means that the crosslinking 



* DNA molecules are not rigid rods and the linear appearance of the 

 untreated DNA is brought about by surface tension during drymg which 

 pulls out the molecules. 



