A sample is placed on a semisolid gel immersed in a solution that conducts electricity.
When current is applied, the
negatively charged DNA fragments
migrate toward the
positive electrode (
The fragments move at rates based mostly on fragment size: smaller molecules migrate at a faster rate through the pores of the medium than larger ones.
The fragments can be visualized as bands by applying dyes or by autoradiography (exposing a radioactive molecule to photographic film).