  The phase diagram of a water solution of DNA and oppositely charged spherical
macroions is studied. DNA winds around spheres to form beads-on-a-string
complexes resembling the chromatin 10 nm fiber. At small enough concentration
of spheres these "artificial chromatin" complexes are negative, while at large
enough concentrations of spheres the charge of DNA is inverted by the adsorbed
spheres. Charges of complexes stabilize their solutions. In the plane of
concentrations of DNA and spheres the phases with positive and negative
complexes are separated by another phase, which contains the condensate of
neutral DNA-spheres complexes. Thus when the concentration of spheres grows,
DNA-spheres complexes experience condensation and resolubilization (or
reentrant condensation). Phenomenological theory of the phase diagram of
reentrant condensation and charge inversion is suggested. Parameters of this
theory are calculated by microscopic theory. It is shown that an important part
of the effect of a monovalent salt on the phase diagram can be described by the
nontrivial renormalization of the effective linear charge density of DNA wound
around a sphere, due to the Onsager-Manning condensation. We argue that our
phenomenological phase diagram or reentrant condensation is generic to a large
class of strongly asymmetric electrolytes. Possible implication of these
results for the natural chromatin are discussed.
