466 



D. O. JORDAN 



Adcnint 



Th/mint 



Cyxot'int 



Fig. 8. The permitted hydrogen bonds in the Watson and Crick formula. 



be either way around, thus adenine can occur in either chain, but when it 

 does, its partner on the other chain must always be thymine. 



The pairing of the bases in this manner is strongly supported by the 

 recent analytical results of Chargaff^'^ and Wyatt.*''*^ The molar ratios of 

 adenine-thymine and guanine-cytosine for a variety of deoxypentose 

 nucleic acids are listed in Chapter 10. These ratios approximate very closely 

 to unity in accordance with the Watson and Crick hypothesis. The ratios 

 of adenine to guanine on the other hand [see Chapter 10] are generally 

 much greater than unity and are variable in different nucleic acids. The 

 formation of hydrogen bonds between the paired bases, as shown in Fig. 

 8, is also in agreement with the titration results of Gulland et al.,*^ who first 

 suggested the formation of hydrogen bonds between the amino and — ^NH — 

 CO — dissociations in order to explain the nonavailabihty of this group 



" E. Chargaff, Experientia 6, 201 (1950); Federation Proc. 10, 654 (1951); for other 



references see Chapter 10 of this book. 

 " G. R. Wyatt, Biochem. J. 48, 584 (1951). 

 " G. R. Wyatt, J. Gen. Physiol. 36, 201 (1952). 



