Microscopics of complexation between long DNA molecules and positively charged colloids.

Extensive atomic force and electron microscopy reveal a new, generic DNA-colloid complex with a fixed number of DNA bases per colloid. The fiber shaped complex is stable in the presence of excess colloids in the solution. As more DNA is added to the solution and the ratio between colloids and DNA approaches the fiber's stoichiometry, the system undergoes a sharp coagulation transition. The system is restabilized at even higher DNA concentrations through localization of small colloid clusters on extensive DNA networks.