gel electrophoresis

Meaning: A gel acts as a sieve. Charged molecules loaded into wells move through its pores under a voltage. When charge per mass is uniform, small molecules travel far and large ones lag near the wells.

Equation: v = qE/f, where v is migration speed (m/s), q is net charge (C), E is electric field strength (V/m), and f is the frictional coefficient (kg/s), set by size, shape and gel pore size.

On the MCAT: Nucleic acids are negative from phosphate groups, so they migrate toward the positive electrode (anode). Questions often estimate fragment length from a ladder of known sizes. In SDS-PAGE, SDS denatures proteins and coats them with negative charge proportional to mass, so mass alone sets migration; β-mercaptoethanol breaks disulfide bonds, so subunits separate. Native PAGE keeps folding, so charge and shape also matter. In isoelectric focusing, each protein stops where pH = pI, its net charge zero.

Common confusion: Students expect larger molecules to go farther. When charge per mass is uniform (DNA fragments, SDS-PAGE), the smaller molecule moves farther, so bands nearest the wells are largest. Shape and charge matter otherwise, as in native PAGE.