PHYSICAL PROPERTIES OF NUCLEIC ACIDS 



465 



34 A 



T 

 3.4 A 



Fig. 7. The helical structure of sodium deoxypentose nucleate proposed by Wat- 

 son and Crick."" 



hydrogen bonds. A single base from one chain is hydrogen-bonded to a 

 single base in the other chain so that the two bases lie side by side with 

 identical ^-coordinates. The formation of these hydrogen bonds between 

 different bases is found, on the basis of molecular models, to be highly 

 specific and only certain pairs of bases will fit into the structure. One mem- 

 ber of a pair must be a purine and the other a pyrimidine in order to bridge 

 between the two chains. A bridge of two pyrimidines is not large enough 

 to form the link and there is not room for two purines. If the most probable 

 tautomeric forms of the purines and pyrimidines are assumed it is then 

 found that the only pairs of bases that are possible are: 



adenine with thymine 



guanine with cytosine 



The ways in which these can be joined are shown in Fig. 8. A given pair can 



