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DNA, the relative magnitudes of the surrounding peaks in these patterns being governed by the binomial-distribution (unpublished observations).