Practice question

Electroendosmosis is bulk fluid flow toward cathode opposite to DNA migration, caused by fixed charges on support matrix. Agarose contains residual sulfate and pyruvate groups introduced during purification from agar, imparting negative charge to gel fibers. Under electric field, counterions in diffuse layer move toward cathode, dragging water with them. This electroosmotic flow retards anionic DNA and causes band broadening. Highly purified low-EEO agarose minimizes this effect. Thus phenomenon originates not from pH or temperature alone, but from intrinsic charged substituents within agarose polymer.