MOLECULAR CONFIGURATION OF NUCLEIC ACIDS 



of one polynucleotide chain and a complementary sequence in another. Such 

 Specific interaction may be the means by which amino acids are attached to 

 the requisite portions of a polynucleotide chain that has encoded in it the 

 sequence of amino acids that specifies a protein. In this case the amino acid is 

 attached to a transfer RNA molecule and part of the polynucleotide chain 

 in this RNA pairs with the coding chain. 



Since the base-pairs were first described by Watson and Crick in 1953, 

 many new data on purine and pyrimidinc dimensions and hydrogen-bond 

 lengths have become available. The most recent refinement of the pairs (due 

 to S. Arnott) is shown in Fig. 2. We now take the distance between Cj atoms 

 as 10.7 A instead of the value used recently of 11.0 A, mainly because new 

 data on N-H...N bonds show that this distance is 0.2 A shorter between 

 ring nitrogen atoms than between atoms that are not in rings. The linearity 

 of the hydrogen bonds in the base-pairs is excellent and the lengths of the 

 bonds are the same as those found in crystals (these lengths vary by about 

 0.04 A). 



The remarkable precision of the base-pairs reflects the exactness of DNA 

 replication. One wonders, however, why the precision is so great, for the 

 energy required to distort the base-pairs so that their perfection is appre- 

 ciably less, is probably no greater than one quantum of thermal energy. The 

 explanation may be that replication is a co-operative phenomenon involving 

 many base-pairs. In any case, it must be emphasized that the specificity of 

 the base-pairing depends on the bonds joining the bases to the deoxyribose 

 groups being correctly placed in relation to each other. This placing is prob- 

 ably determined by the DNA polymerizing enzyme. Whatever the me- 

 chanics of the process are, the exact equivalence of geometry and envi- 

 ronment of every nucleotide in the double-helix should be conducive to 

 precise replication. Mistakes in the copying process will be produced if there 

 are tautomeric shifts of protons involved in the hydrogen-bonding or chem- 

 ical alterations of the bases. These mistakes can correspond to mutations. 



The Universal Nature and Constancy of the Helical Structure of DNA 



After our preliminary X-ray studies had been made, my friend Leonard 

 Hamilton sent me human DNA he and Ralph Barclay had isolated from 

 human leucocytes of a patient with chronic myeloid leukaemia. He was 

 studying nucleic acid metabolism in man in relation to cancer and had pre- 



s-40 



