DNA that is cut by restriction enzymes will always produce the same fragments, giving each person a unique DNA fingerprint that can be read using a technique called gel electrophoresis.
A DNA fingerprint is a nearly unique set of genetic characteristics, as identified through a collection of DNA restriction fragments. Though television crime dramas give an unrealistic portrayal of the amount of time required for DNA analysis, DNA evidence is extremely useful in placing certain individuals at crime scenes. This is because each individual has a unique DNA sequence, and certain parts of the genome are highly variable among individuals. DNA evidence collected from a crime scene can be exposed to restriction enzymes, which cut DNA at a specific sequence and are designed to target some of those highly variable regions of DNA. The resulting mixture of DNA fragments is then subjected to gel electrophoresis, a technique that separates DNA and other molecules based on their size using electric charge. This technique separates the DNA into a unique pattern of bands visible in agar (a gelatinous substrate for growing biological specimens). It uses an electrical field to separate large molecules based on their size and whether they are negatively or positively charged. The same person will yield the same pattern every time that a particular restriction enzyme is used. Similar technology is employed in cases of questioned parentage. A sample of the mixture of DNA fragments is added to wells in one end of an agar-gel sheet that floats in an electrolyte solution between two electrodes. When an electric current is passed through the bath, the DNA fragments move toward the positive end of the chamber. Smaller pieces of DNA are able to move through the gel more easily because they have less resistance than large pieces. After some time, separate bands appear on the gel where the small pieces of DNA have moved further ahead and longer fragments have lagged behind. The DNA of multiple sources can be run through the same agar-gel sheet for easy comparison. If the bands match, the conclusion is that the DNA sample came from the same person.