To separate samples of DNA that have been digested by restriction enzymes, you will need to use gel electrophoresis. Gel electrophoresis is a procedure that separates molecules based on their rate of movement through a gel under the influence of an electrical field created by a negative electrode on one side of the gel and a positive electrode on the other. The gel is made of agarose, an agar-like material that can be prepared in different concentrations, much like firm Jello and wiggly Jello. The direction of movement of the molecules being separated is affected by the charge of the molecules, and the rate of movement is affected by their size and shape, the density of the gel, and the strength of the electrical field. Specific agarose concentrations are used to separate different fragment sizes. DNA is a negatively charged molecule, so it will move toward the positive pole of the gel when a current is applied. DNA fragments migrate at different rates relative to their size. The smallest fragments move through the gel more easily and so migrate the farthest during the time the current is on, whereas the largest fragments move more slowly and appear closer to the negative electrode. Thus the positions of the DNA fragments, which appear as bands in the gel, indicate the size of the fragments relative to each other. Keep in mind that the length (size) of each fragment is measured in number of DNA base pairs.