DNA is a charged molecule. Consequently, DNA molecules will move when an electrical field is applied to a liquid in which they are dissolved. If the liquid is a simple one--such as water with some salts in it--all the DNA molecules move at nearly the same speed. Under those conditions, it is hard to distinguish the tiny disparities in the motion of different kinds of DNA.
"If the solution is made less liquid, as in a gel, and the DNA molecules all start moving across the solution from some initial small volume--that is, from essentially the same staring point--then the molecules can move at perceptibly different speeds. Usually smaller DNA molecules move faster than larger ones. After a while, the molecules are separated by size. If the molecules fall into only a few discreet sizes, then bands (little rectangles) of DNA will appear in the gel. Each of these bands contains DNA strands of a specific size."
[Editors note: DNA fingerprinting uses gel electrophoresis to distinguish between samples of the genetic material. The human DNA molecules are treated with enzymes that chop them at certain characteristic points, thereby reducing the DNA to a collection of more manageably sized pieces. The DNA fragments are loaded into a gel and placed in an electrical field, which electrophoretically sorts the DNA fragments into various bands. These bands can be colored with a radioactive dye to make them visible to imaging techniques.]
