DNA test in a criminal investigation

The collected cells are broken open and the DNA is separated into single strands.

Picture of preparing DNA sample for fingerprinting

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 indicate that the suspect is not the criminal.

Last Updated: April 20, 2009

Medical Review: Joy Melnikow, MD, MPH - Family Medicine & Siobhan M. Dolan, MD, MPH - Reproductive Genetics

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