THE STRUCTURE OF DNA 



possibility might be that the long strands of right handed DNA are joined together 

 by compensating strands of left handed polypeptide helices. The merits of this 

 proposition are difficult to assess, but the fact that the phage DNA does not seem 

 to be linked to protein makes it rather unattractive. 



The untwisting process would be less complicated if replication started at the 

 ends as soon as the chains began to separate. This mechanism would produce 

 a new two-strand structure without requiring at any time a free single-strand 

 stage. In this way the danger of tangling would be considerably decreased as 

 the two-strand structure is much more rigid than a single strand and would 

 resist attempts to coil around its neighbors. Once the replicating process is 

 started the presence, at the growing end of the pair, of double-stranded structures 

 might facilitate the breaking of hydrogen bonds in the original unduplicated 

 section and allow replication to proceed in a zipper-like fashion. 



It is also possible that one chain of a pair occasionally breaks under the strain 

 of twisting. The polynucleotide chain remaining intact could then release the 

 accumulated twist by rotation about single bonds and following this, the broken 

 ends, being still in close proximity, might rejoin. 



It is clear that, in spite of the tentative suggestions we have just made, the 

 difficulty of untwisting is a formidable one, and it is therefore worthwhile re- 

 examining why we postulate plectonemic coiling, and not paranemic coiling in 

 which the two helical threads are not intertwined, but merely in close apposition 

 to each other. Our answer is that with paranemic coiling, the specific pairing of 

 bases would not allow the successive residues of each helix to be in equivalent 

 orientation with regard to the helical axis. This is a possibility we strongly 

 oppose as it implies that a large number of stereochemical alternatives for the 

 sugar-phosphate backbone are possible, an inference at variance to our finding, 

 with stereochemical models (Crick and Watson, 1953) that the position of the 

 sugar-phosphate group is rather restrictive and cannot be subject to the large 

 variability necessary for paranemic coiling. Moreover, such a model would not 

 lead to specific pairing of the bases, since this only follows if the glucosidic links 

 are arranged regularly in space. We therefore believe that if a helical structure 

 is present, the relationship between the helices will be plectonemic. 



We should ask, however, whether there might not be another complementary 

 structure which maintains the necessary regularity but which is not helical. 

 One such structure can, in fact, be imagined. It would consist of a ribbon-like 

 arrangement in which again the two chains are joined together by specific pairs 

 of bases, located 3.4 A above each other, but in which the sugar-phosphate 

 backbone instead of forming a helix, runs in a straight line at an angle approx- 

 imately 30° off the line formed by the pair of bases. While this ribbon-like 

 structure would give many of the features of the X-ray diagram of Structure B, 

 we are unable to define precisely how it should pack in a macroscopic fiber, 

 and why in particular it should give a strong equatorial reflexion at 20-24 A. 

 We are thus not enthusiastic about this model though we should emphasize 

 that it has not yet been disproved. 



Independent of the details of our model, there are two geometrical problems 



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