Gel electrophoresis

In gel electrophoresis, samples are loaded into small wells at one end of a slab of gel, and an electric current is passed through it. DNA and RNA carry a negative charge on their phosphate backbones, so they travel toward the positive electrode. The gel is a mesh of tiny pores that works like a sieve: small molecules slip through quickly while large ones lag behind, so after a while the molecules are spread out by size.

Agarose, a substance purified from seaweed, is the usual gel for DNA fragments. Polyacrylamide gives finer separation and is used for proteins and very small nucleic acids; proteins are often coated with the detergent SDS first, so they all carry negative charge and separate mainly by size, a method called SDS-PAGE. A ladder of fragments of known size runs beside the samples, and a dye makes the molecules visible as bands whose sizes can be read against the ladder.

Students use it to check PCR products and to see how restriction enzymes cut a plasmid. Early electrophoresis helped Linus Pauling and colleagues show that sickle cell hemoglobin carries a different charge from normal hemoglobin.