gel electrophoresis

Imagine a crowd running through thick mud toward an exit: small, light people slip through quickly while big, heavy ones lag behind, so after a while the runners are spread out by size. Gel electrophoresis uses this idea to sort molecules. It places a mixture of DNA fragments (or proteins) into a slab of jelly-like gel and pulls them through it with electricity, so that, by the end, the molecules are separated into bands according to their size.

DNA carries a negative electric charge, so when an electric field is applied across the gel, all the fragments crawl toward the positive end. The gel is a mesh of tiny pores, like a microscopic obstacle course: small fragments wriggle through easily and travel far, while large fragments snag and move slowly, staying near the start. Identical-sized fragments pile up together into a visible band. By running known size markers alongside, you can read each band's size, and a stain or fluorescent dye makes the bands glow so you can photograph them.

Gel electrophoresis is a staple of nearly every molecular biology lab. It is the standard way to check whether a PCR worked, to estimate the size of DNA fragments, to separate proteins by size as the first step of a Western blot, and to read DNA before sequencing. Its job is separation and sizing, not identification: a band tells you a molecule of that size is present, but not what its sequence is or what it does — you need further tests, such as sequencing or antibody probing, to learn that.