SDS-PAGE

Meaning: SDS is an anionic detergent that denatures proteins and binds them in roughly constant amounts per gram. This masks intrinsic charge, so proteins have a similar charge-to-mass ratio. In the polyacrylamide gel, a sieve, smaller proteins move faster and farther toward the anode. A reducing agent such as β-mercaptoethanol or DTT breaks disulfide bonds and separates subunits.

Equation: log₁₀(M) = a − b·d, where M is molecular mass (Da; 1 Da = 1 g/mol), d is distance migrated from the well (m), and a and b are constants fitted from standards run on the same gel.

On the MCAT: A passage shows a stained gel with a ladder of standards. You estimate an unknown's mass from its band position, or infer subunit makeup. For example, one band at 50 kDa without reducing agent that becomes a 25 kDa band with it suggests two identical subunits joined by disulfide bonds. Native PAGE is the contrast case: no SDS, so structure is kept and separation depends on charge, size and shape.

Common confusion: Students say SDS-PAGE separates by charge because electrophoresis usually does. Here SDS makes charge roughly uniform, so mass sets the order. Masses are estimates, since some proteins (heavily glycosylated ones, for example) run anomalously. Bands report subunit masses, not the mass of the intact complex.