Electrophoresis
Electrophoresis is a technique that separates molecules such as DNA, RNA and proteins based on size and charge. A common form of electrophoresis, called agarose gel electrophoresis, is demonstrated in this animation.
In this technique, DNA of different sizes is first loaded into the 'well' of an agarose gel. Agarose is a thick, jello like substance that is made from seaweed. Wells are produced by melting the agarose, and then allowing it to solidify around a mold that creates a rectangular hole in the top of the gel. Once the DNA is loaded into the well, an electrical charge is applied. Since the phosphates in a DNA molecule are negatively charged, DNA will move towards the positive charge on the far side of the gel. As they move through the gel matrix, the large pieces of DNA literally have a tighter squeeze than the small fragments and thus move more slowly. Thus, when the gel has finished running, each DNA fragment will be separated by size, with the smallest fragments located farthest from the wells.