DNA Fingerprinting in a Criminal Investigation

The first step in DNA fingerprinting is to break open the sample cells and collect the DNA. Next, many copies of the DNA are made using polymerase chain reaction (PCR).

To get the DNA fingerprint:

• All of the DNA is broken into pieces at certain locations. This is done by chemicals (enzymes) that break each DNA strand at the same place.
• The DNA pieces are placed at the top of a special gel held in a frame.
• An electric current is applied to the gel. The current separates the DNA into bands of identical pieces. This process is called electrophoresis.
• The band pattern is transferred to a nylon membrane containing a radioactive chemical. The chemical makes the bands show up clearly.

For a criminal investigation, photos of the criminal and suspect DNA are compared. The results in this case show that the suspect is not the criminal.