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



chain is in the form of a regular helix. Thus, irrespective of which bases are 

 present, the glucosidic bonds (which join sugar and base) are arranged in a regu- 

 lar manner in space. In particular, any two glucosidic bonds (one from each 

 chain) which are attached to a bonded pair of bases, must always occur at a fixed 

 distance apart due to the regularity of the two backbones to which they are joined. 

 The result is that one member of a pair of bases must always be a purine, and 

 the other a pyrimidine, in order to bridge between the two chains. If a pair con- 

 sisted of two purines, for example, there would not be room for it; if of two 

 pyrimidines they would be too far apart to form hydrogen bonds. 



In theory a base can exist in a number of tautomeric forms, differing in the 

 exact positions at which its hydrogen atoms are attached. However, under physi- 

 ological conditions one particular form of each base is much more probable than 

 any of the others. If we make the assumption that the favored forms always oc- 

 cur, then the pairing requirements are even more restrictive. Adenine can only 

 pair with thymine, and guanine only with cytosine (or 5-methyl-cytosine, or 5- 

 hydroxy-methyl-cytosine). This pairing is shown in detail in Figures 5 and 6. If 

 adenine tried to pair with cytosine it could not form hydrogen bonds, since there 

 would be two hydrogens near one of the bonding positions, and none at the other, 

 instead of one in each. 



A given pair can be either way round. Adenine, for example, can occur on 

 either chain, but when it does its partner on the other chain must always be 

 thymine. This is possible because the two glucoside bonds of a pair (see Figures 5 

 and 6) are symmetrically related to each other, and thus occur in the same po- 

 sitions if the pair is turned over. 



It should be emphasized that since each base can form hydrogen bonds at a 

 number of points one can pair up isolated nucleotides in a large variety of ways. 

 Specific pairing of bases can only be obtained by imposing some restriction, and 

 in our case it is in a direct consequence of the postulated regularity of the phos- 

 phate-sugar backbone. 



It should further be emphasized that whatever pair of bases occurs at one par- 

 ticular point in the DNA structure, no restriction is imposed on the neighboring 

 pairs, and any sequence of pairs can occur. This is because all the bases are flat, 

 and since they are stacked roughly one above another like a pile of pennies, it 

 makes no difference which pair is neighbor to which. 



Though any sequence of bases can fit into our structure, the necessity for spe- 

 cific pairing demands a definite relationship between the sequences on the two 

 chains. That is, if we knew the actual order of the bases on one chain, we could 

 automatically write down the order on the other. Our structure therefore consists of 

 two chains, each of which is the complement of the other. 



IV. Evidence in Favor of the Complementary Model 

 The experimental evidence available to us now offers strong support to our 



model though we should emphasize that, as yet, it has not been proved correct. 



The evidence in its favor is of three types: 



(1) The general appearance of the X-ray picture strongly suggests that the 



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