Orateur
Description
DNA is a negatively charged semi-flexible polymer, which forms extended coils in water, due to the electrostatic repulsions between its negatively charged segments and its bending rigidity. However, in vivo, it is tightly packed and curved. The most extreme confinements are achieved through hexagonal arrays found in vivo in some viral capsids or spermatozoa. Curved and dense arrays also spontaneously form in vitro upon DNA condensation: in the presence of multivalent cation, basic proteins or crowding agents, long DNA molecules collapse in the form of compact toroidal globules.
We use cryo electron microscopy to analyse the structure of curved DNA curved hexagonal arrays in DNA toroids and in bacteriophage capsids. In both cases, curved DNA hexagonal arrays are complex liquid crystalline objects are where topological constraints imply the presence of defects. Beyond the simple cross over due to the continuity of the chain, the reversal of the folding direction and the competition between twist induced by DNA helicity and the parallelism of the hexagonal order results in polymorphic geometries. The analysis of DNA local order with groove correlations sheds light on the interaction between highly curved helices at high density and reveals curvature-dependent inter-helix interactions.
