The most popular variant of electrophoresis is carried out with agarose gels. When cooled from heated solutions, solutions of agarose form a nearly transparent gel that looks like a slab of gelatin dessert. After the gel sets, DNA solutions are placed in little depressions formed by leaving a comb in the gel. The gel is then flooded with a weak salt solution, and a voltage is applied. Large DNA molecules advance slowly in the gel because they are impeded by the gel matrix.
Smaller ones encounter fewer barriers and move more quickly.
From these data, the length of unknowns can be easily calculated. For ease of comparison, the standards are usually run in the same gel beside the unknowns. Gels of differing porosities can be made by adjusting the concentration of agarose. The picture above, that at the right, and photograph of the gel below, come from BioRad 's web site. BioRad is a manufacture and distributor of Life Sciences Research Products.