Gel electrophoresis is a method to separate and view macromolecules (large molecules, such as DNA, RNA and proteins). Gel electrophoresis used in forensic science is a way to analyze DNA. Because the DNA of each person is unique, the patterns of separation created using gel electrophoresis are unique. In the case of a crime, the forensic scientist can perform an electrophoresis of DNA from the crime scene and from the suspect's DNA, then determine if the two are an exact match.
Cutting the DNA
Intact DNA is so large that it cannot move through a gel, so the forensic scientist must first cut the DNA into smaller pieces using restriction enzymes. Restriction enzymes recognize specific sequences of the building blocks of the DNA and cut the DNA into these sequences. The scientist uses these cut sequences to make DNA solutions (mixtures of different sizes of DNA fragments).
To perform gel electrophoresis, the scientist first prepares a gel, usually made from agarose, a substance derived from seaweed. After the gel is set, holes are made in the gel to hold the DNA solution. Then the scientist places the DNA solution in the gel and applies an electrical current to the gel.
DNA Movement and Pattern
The electrical current causes the DNA fragments to move. Because DNA is negatively charged, it moves away from the negative end of the electrical field and toward the positive end. But different fragments move at different speeds, creating a unique separation pattern. To see the pattern, the forensic scientist adds either dyes to stain the area of the gel that contains the DNA or chemicals to bind the macromolecules and cause them to give off a fluorescent glow under ultraviolet light.
Purpose of the Gel
The gel serves two main purposes. First, it is a way to hold the DNA and know where they are. Second, the gel has many microscopic holes for the DNA to move through as they migrate within the electric field. The smaller the DNA sequence, the easier it moves through the holes, which means smaller pieces of DNA move through the gel faster than larger pieces.
Forensic science also uses gel electrophoresis to separate DNA fragments so that the individual fragments can be analyzed using other forensic techniques.