Gel electrophoresis.

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 (anode).

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).