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Dense phases of DNA and nucleosomes

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Françoise Livolant
Amélie Leforestier

We design in vitro simplified model systems of condensed DNA and chromatin. We explore their physico-chemical properties and establish phase diagrams to understand their self-assembly properties.

  • Nucleosomes Nucleosome core particles are formed by a segment of 146 bp DNA coiled into a left handed superhelix around the histone core. We analyse how these particles self organise under physico-chemical conditions that reproduce those found in the living cell. We vary different parameters : (i) the osmotic pressure that allows us to control precisely the nucleosome concentration within a biologically relevant range (100-500 mgml), and (ii) the monovalent salt concentration (3.5 to 160 mM), (iii) the presence of di or multivalent cations.   In the presence of monovalent counterions, we have established a phase diagram : above a critical osmotic pressure, nucleosome stack on top of each other to form columns. These columns further organise into multiple crystalline or liquid crystalline phases. A lamello-columnar and an inverse hexagonal phase form under low salt conditions. A 2D hexagonal and a 3 D orthorhombic phase are found at higher salt concentrations. These structures are characterised using X-ray scattering and cryo-electron microscopy and can be related to the complex poly-ampholyte characteristic of the nucleosome. All these phases also present complex mesoscopic chiral organisations. We analyse their multiple levels of helicity.

    Phase diagram of Nucleosome Core Particles in solution in the presence of monovalent counterions

    Multiple phases of Nucleosome Core Particles observed in polarizing microscopy


  • DNA liquid crystals

As first described by C. Robinson (1961, 1966) and V. Luzzati (1959, 1963), DNA in aqueous solution forms liquid crystalline phases, whose nature depends on the DNA and monovalent ion concentrations. We showed that 50 nm DNA fragments (146 bp) form a variety of liquid cryslline phases (blue phases, cholesteric phase, hexagonal phase) or 3D crystals.

DNA cholesteric phase observed in the polarizing microsocope and in the electron microsope, after freeze-fracturing

DNA columnar hexagonal phase observed in polarizing microscopy and in the electron microsocpe after freeze-fracturing



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