"The configuration of the nucleic acids"

Date:
1955
Reference:
PP/CRI/H/1/39
Catalogue details

Licence: In copyright

Credit: "The configuration of the nucleic acids". Source: Wellcome Collection.

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    3, The phosphates are near the outside of the structure, the bases nearer in, !+» The chains are held together by hydrogen bonds betwea^t specific pairs of bases. We shall now indicate the way in which the experimental evidence supports these features. It is easiest to take point No,2 first. The X-ray pattern clearly establishes that the structure is helical, with many residues per turn. The observed absences could scarcely be due to any other reason. Any lingering doubts are removed by the n cylindrical nature of the packing of the unit cell, and the approximation of the diffraction pattern to cylindrical symmetry (this point was not clear from the earlier work). The argument which suggests that there are two chains is more o complicated. The strong 3»4 A reflection on or near the meridian in lattice B can only be due to the bases, and showns that there are n nucleotides per lattice point per 3»4 A in the fibre direction, where n is an integer. Density arguments show that n » 1 and n = 4 are impossible, so that n » 2 or » 3, Since the fibre shrinks in length by $0$ in going to lattice A the latter must have n residues in 3»b/l*3 = 2,6 A. The density of lattice A, which can be measured with more precision than the less well-defined lattice B, shows unabiguosly that n » 2, This does not of itself show that there are two chains, but it restricts the choice to one, two or four chains. Four chains, with a nucleotide every 6,8 2. in the fibre direction, and staggered to give the 3.4 S reflection, can be ruled out, since such a structure could not be reversibly extended by 50$ because the maximum possible phosphate to phosphate diBtance