HENRY M. SOBELL
Perspective illustration of the left-handed toroidal structure, calculated by placing a low energy beta-structural element every 10 base-pairs along B-DNA. Each residue of the helix contains 10 base-pairs -- the helix being generated from this residue by a twist of -41.1 degrees and a translation along the helix axis of 5.26 Angstroms. The structure has a diameter of 100 Angstroms and contains about one and one-half turns per 140 base-pairs. The long central line indicates the super-helix axis – the length shown is 90 Angstroms. This basic structure could be used in the organization of DNA within the nucleosome.
(Redrawn, from Sobell, H. M. et al, 1976)
NOTE: If, however, there were an equal probability that either 11 base-pairs of A-DNA or 10 base-pairs of B-DNA exist within any given segment of the left-handed toroidal super-helical structure shown above, a population of such aperiodic structures can give rise to the periodic cutting-patterns observed experimentally (it is interesting to note in this regard that an A-DNA allomorph is known that contains 11 base-pairs per turn in 34.0 Angstroms (Arnott, S., Chandrasekaran, R., Hall, I.H., Puigjaner, L.C., Walker, J.K., and Wang, M. Cold Spring Harbor Symp Quant Biol 47, 53-65 (1983)). This would be true for naked DNA molecules immobilized on a calcium-phosphate crystalline-surface as well, provided they also form left-handed toroidal super-helices under such conditions. In both cases, probability-considerations predict cutting patterns to be symmetrically distributed around integral multiples of 10.5 base-pairs along DNA, the relative magnitudes of the surrounding peaks in these patterns being governed by the binomial-distribution (unpublished observations).