The dissolution of either crystalline, ultrapure urea or a concentrated solution of ultrapure urea is an endothermic process. Although the solution will cool noticeably and the viscosity will increase, the solution will not gel. The urea prevents the formation of ice crystals. If a satisfactory gel cannot be obtained by the above procedures, cast an agarose gel in the normal manner, then equilibrate in a urea solution prepared from a buffer or other solute. Avoid exposing urea to high temperatures.
Urea solutions decompose to ammonia and biuret with excessive heat.
Cyanate formation of other gel components may also occur. The optimal gel concentration to prepare depends on the size of the DNA fragments to be resolved. The distance used to determine voltage gradients is the distance between the electrodes, not the gel length. Buffer also plays a role in band sharpness. The tables below provide a quick reference for optimal voltage to run DNA and the recommended voltages for BioWhittaker Molecular Applications agaroses.