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).