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Stretched and overwound DNA forms a Pauling-like structure with exposed bases
J F Allemand1, D Bensimon, R Lavery
1Laboratoire de Physique Statistique, Ecole Normale Supérieure, Unité de Recherche Associée D 1306 Centre National de la Recherche Scientifique, Paris Cedex 05, France.allemand@physique.ens.fr
Summary
Stretched DNA molecules exhibit structural transitions. Both negatively and positively supercoiled DNA show coexistence with other forms, including a novel, highly twisted DNA structure under positive supercoiling.
Area of Science:
- Molecular Biology
- Biophysics
- Structural Biology
Background:
- DNA structure is typically B-DNA, but can transition under stress.
- Supercoiling and stretching forces significantly influence DNA conformation.
Purpose of the Study:
- To investigate structural transitions in single DNA molecules under varying supercoiling and stretching forces.
- To characterize novel DNA structures formed under these conditions.
Main Methods:
- Single-molecule DNA stretching experiments.
- Varying supercoiling densities (sigma).
- Molecular modeling and simulations.
Main Results:
- Negative supercoiling (>0.3 pN) induces coexistence of B-DNA and denatured DNA.
- Positive supercoiling (3 pN) reveals coexistence of B-DNA and a new, highly twisted structure.
- The novel structure has ~2.62 bases/turn and is 75% longer than B-DNA, with interwound backbones and exposed bases.
Conclusions:
- DNA exhibits complex structural plasticity beyond B-DNA under mechanical stress.
- The newly identified twisted structure shares features with historical DNA models and bacteriophage structures.
- Mechanical forces can drive DNA into non-canonical conformations relevant to biological processes.