Gel electrophoresis separates charged biomolecules — DNA, RNA, or SDS-coated proteins — by forcing them through a porous gel matrix under an electric field. Samples are loaded into wells cut near the negative electrode (cathode). Because nucleic acids carry a uniform negative charge along their backbone (and SDS gives denatured proteins a uniform negative coat), every fragment is pulled toward the positive electrode (anode) — but the gel's mesh of polymer fibres acts like a sieve, slowing large molecules more than small ones.
Because migration distance depends logarithmically on fragment size, gel electrophoresis can't distinguish very large fragments well — above roughly 20–50 kb, DNA of any size barely moves through a standard agarose gel, which is why pulsed-field electrophoresis was invented for whole chromosomes.
A glowing 3D gel slab sits between two electrodes. Load a DNA digest or an SDS-PAGE protein sample into the wells, apply a voltage, and watch each fragment migrate toward the anode at a rate set by its own size and the density of the gel matrix.
Charged fragments are pulled through a porous gel by an electric field; the sieving mesh slows large molecules far more than small ones, so a mixed sample resolves into separate bands ordered by size — smallest travels farthest, largest barely moves.
Set the voltage and gel density, pick a DNA or protein sample, then press Run. Watch the reference ladder and sample lanes spread out over time, and check the leading band's Rf value as it approaches the anode.
SDS-PAGE works because sodium dodecyl sulfate coats denatured proteins in a uniform negative charge-to-mass ratio, so — unlike native proteins — they separate almost purely by size, just like DNA fragments do in an agarose gel.