%^^c-c 



/ " THYMINE GUANINE ^^,h ^ CTTO I— 



Fig. I. Hydrogen Bonding of Bases. 



However, there is an equivalence of the bases with an amino group in the 

 6-position of the ring, to the bases with a keto group in the 6-position. These 

 facts were interpreted by Watson and Crick (5) in their masterful hypothesis 

 on the structure of DNA. As shown in Fig. i, they proposed in connection 

 with their double-stranded model for DNA, to be discussed presently, that 

 the 6-amino group of adenine is linked by hydrogen bonds to the 6-keto group 

 of thymine and in a like manner guanine is hydrogen-bonded to cytosine, thus 

 accounting for the equivalence of the purines to the pyrimidines. On the basis 

 of these considerations and the results of X-ray crystallographic measurements 

 by WiLKiNS and associates (6), Watson and Crick proposed a structure for 

 DNA in which two long strands are wound about each other in a helical 

 manner. Fig. 2 is diagrammatic representation of a fragment of a DNA chain 

 about ten nucleotide units long. According to physical measurements, DNA 

 chains are on the average 10 000 units long. We see here the deoxypentose 

 rings linked by phosphate residues to form the backbone of the chain; the 

 purine and pyrimidine rings are the planar structures emerging at right angles 

 from the main axis of the chain. Fig. 3 is a more detailed molecular model (7) 

 and gives a better idea of the packing of the atoms in the structure. The 

 purine and pyrimidine bases of one chain are bonded to the pyrimidine and 

 purine bases of the complementary chain by the hydrogen bonds described 

 in Fig. I. The X-ray measurements have indicated that the space between 

 the opposing chains in the model agrees with the calculated value for the 

 hydrogen-bond linkage of a purine to a pyrimidine; it is too small for two 

 purines and too large for two pyrimidines. Most rewarding from the biological 

 point of view, the structure provides a useful model to explain how cellular 

 replication of DNA may come about. For, if you imagine that these two 

 chains separate and that a new chain is formed complementary to each of 

 them, the result will be two pairs of strands, each pair identical to the original 

 parent duplex and identical to each other. 



S-51 



