Why can gel electronphoresis separate DNA fragments

DNA is negatively charged and therefore moves toward the positive electrode under the influence of an electric current. Smaller fragments migrate toward the positive electrode at a faster pace than do larger fragments. An electric current separates different-sized molecules in a sponge-like matrix because the molecules go towards whichever pole is the opposite charge of their own. The smaller, lighter molecules move through the gel's pores more easily than the large heavy molecules, therefore going further.