242 



is not clear what forces would hold 

 the structure together, especially as 

 the negatively charged phosphates 

 near the axis will repel each other. (2) 

 Some of the van der Waals distances 

 appear to be too small. 



Fig. 1. This figure is purely diagram- 

 matic. The two ribbons symbolize the 

 two phosphate-sugar chains, and the hori- 

 zontal rods the pairs of bases holding the 

 chains together. The vertical line marks 

 the axis. 



Another three-chain structure has 

 also been suggested by Fraser (in the 



WATSON AND CRICK 



press). In his model the phosphates are 

 on the outside and the bases on the 

 inside, linked together by hydrogen 

 bonds. This structure as described is 

 rather ill-defined, and for this reason 

 we shall not comment on it. 



We wish to put forward a radically 

 different structure for the salt of de- 

 oxyribose nucleic acid. This structure 

 has two helical chains each coiled 

 round the same axis (see diagram). We 

 have made the usual chemical assump- 

 tions, namely, that each chain consists 

 of phosphate diester groups joining 

 ^-D-deoxyribofuranose residues with 

 3', 5' linkages. The two chains (but not 

 their bases) are related by a dyad per- 

 pendicular to the fibre axis. Both 

 chains follow right-handed helices, but 

 owing to the dyad the sequences of 

 the atoms in the two chains run in 

 opposite directions. Each chain loosely 

 resembles Furberg's (1952) model No. 

 1; that is, the bases are on the inside 

 of the helix and the phosphates on the 

 outside. The configuration of the sugar 

 and the atoms near it is close to Fur- 

 berg's 'standard configuration,' the 

 sugar being roughly perpendicular to 

 the attached base. There is a residue 

 on each chain every 3.4 A. in the z-di- 

 rection. We have assumed an angle of 

 36° between adjacent residues in the 

 same chain, so that the structure re- 

 peats after 10 residues on each chain, 

 that is, after 34 A. The distance of a 

 phosphorus atom from the fibre axis 

 is 10 A. As the phosphates are on the 

 outside, cations have easy access to 

 them. 



The structure is an open one, and its 

 water content is rather high. At lower 

 water contents we would expect the 

 bases to tilt so that the structure could 

 become more compact. 



The novel feature of the structure 

 is the manner in which the two chains 

 are held together by the purine and 

 pyrimidine bases. The planes of the 



