J. D. WATSON AND F. H. C. CRICK 



ADENINE 



THYMINE 



ADENINE 



CYTOSINE 



Figure 7. Pairing arrangements of adenine before (above) and after (below) 

 it has undergone a tautomeric shift. 



which any model for DNA must face. Both involve the necessity for some form 

 of super folding process and can be illustrated with bacteriophage. Firstly, the 

 total length of the DNA within T2 is about 8 X lO^A. As its DNA is thought 

 (Siegal and Singer, 1953) to have the same very large M. W. as that from other 

 sources, it must bend back and forth many times in order to fit into the phage 

 head of diameter 800 A. Secondly, the DNA must replicate itself without getting 

 tangled. Approximately 500 phage particles can be synthesized within a single 

 bacterium of average dimensions 10^ X 10^ X 2 X lO"* A. The total length of 

 the newly produced DNA is some 4 X 10^ A, all of which we believe was at some 

 interval in contact with its parental template. Whatever the precise mechanism 

 of replication we suspect the most reasonable way to avoid tangling is to have 

 the DNA fold up into a compact bundle as it is formed. 



A Possible Mechanism for Natural Mutation 



In our duplication scheme, the specificity of replication is achieved by means 

 of specific pairing between purine and pyrimidine bases; adenine with thymine, 



206 



