Agarose gel electrophoresis of an E. coli DNA preparation

Gel electrophoresis can be used to separate DNA fragments of different sizes. First a gel is cast from agarose. At one end of the gel are several small wells, made by the teeth of a comb that was placed in the gel before it set. A buffer solution is poured over the gel, so that it fills the wells and makes contact with the electrodes at each end of the gel tank.

The DNA fragments are mixed with a small amount of loading dye. This allows the DNA to sink into the wells. An electrical current is applied to the electrodes and the DNA being negatively charged, migrates to the positive electrode through the gel. Small fragments move quickly through the gel, thus further. Large fragments travel more slowly, thus the DNA fragments are separated by virtue of their size.