How does gel electrophoresis separate fragments of DNA?

There is a specific density of gel used in the electrophoresis. The DNA is placed in a well, and then electrical charge is used to pull the DNA through the gel. Because spliced DNA is slightly charged, it begins to move through the gel. The density of the gel causes the larger pieces to go slower than the smaller pieces. Think of it like this: what is a faster way to get through rush hour traffic? Using a bicycle to pedal through all the cars, or being stuck in a taxi cab. The taxi cab, which is larger, moves slower through the traffic. The bicycle which is smaller, moves quicker.