Liquid-crystalline DNA phases

(A) A liquid-crystalline phase of purified DNA molecules that are observed by polarized-light microscopy. This ordered phase is spontaneously formed once the DNA concentration exceeds a threshold value. (B) Electron micrograph of starved Dps- Escherichia coli cells, which shows a cholesteric liquid-crystalline DNA phase. DNA (in the form of nested arcs that are characteristic of a cholesteric phase) and ribosomes, which appear as dark particles at the cell periphery, are clearly phase-separated. The scale bar is 150 nm. (C) Schematic representation of a cholesteric liquid-crystalline phase. In this organization, chiral rod-like species such as DNA molecules are partially aligned in successive layers that continuously rotate with respect to each other to form a helical arrangement. When this organization is sliced at an oblique angle to the cholesteric axis (perpendicular to the plane), nested arc patterns reminiscent of those shown by DNA in starved Dps- E. coli cells (b) are obtained (Oxford University Press 2001).