When does an analyte run through a gel matrix at constant velocity, according to Stokes Law?

The correct choice focuses on the balance between the electrophoretic driving force and the frictional force acting on the analyte when it travels through a gel matrix. According to Stokes Law, the velocity of a particle moving through a viscous medium is determined by the balance between the forces acting on it.

When an analyte is subjected to an electric field, it experiences a driving force that propels it forward. Simultaneously, as it moves through a gel matrix, it encounters resistance in the form of friction from the gel. For the analyte to run through the gel matrix at a constant velocity, these two forces must be equal. This means that the force pushing the analyte forward due to the electric field must precisely counteract the resistance caused by the gel. If this balance is achieved, the analyte will move at a steady speed, indicating a dynamic equilibrium between the opposing forces.

Other options imply scenarios that do not facilitate the condition for constant velocity. For instance, dilution of the sample or changing the conditions of the gel concentration or electric field may affect the interactions but do not address the critical balance of forces as described by Stokes Law.