Document Type


Publication Date


Publication Title

Proceedings of the National Academy of Sciences of the United States of America


Department of Chemistry


Spermidine-condensed phi X174 replicative form (RF) II DNA was digested with micrococcal nuclease to yield seven identifiable DNA bands forming an arithmetic fragment-length series. The DNA monomer unit length was found to be 780 +/- 80 base pairs. This result is most consistent with a proposed model for micrococcal nuclease cleavage of a DNA torus organized by the unidirectional, circumferential wrapping of B-geometry DNA. By a topological consideration, the blunt-end-rod-fusion model for torus formation [Eickbush, T. H. & Moudrianakis, E. N. (1978) Cell 13, 295-306] is shown to be inconsistent with our empirical solution results. We propose a continuous, circumferential DNA wrapping model in which a significant fraction of the collapsed circular phi X174 RFII DNA molecules form regular toruses comprised of seven complete, unidirectional double-helical wraps.

Original Citation

Marx KA, Reynolds TC. Spermidine-condensed phi X174 DNA cleavage by micrococcal nuclease: torus cleavage model and evidence for unidirectional circumferential DNA wrapping. Proc Natl Acad Sci U S A. 1982;79(21):6484-6488. doi:10.1073/pnas.79.21.6484