Many people know about DNA Fingerprinting, but very few know how the process works. Here, you will learn what scientists must do to determine if a person and a DNA sample match.
Fingerprinting a Crime Suspect
There are three basic components to a DNA fingerprint test. First, there is the DNA from the crime scene; second, there is the DNA from a suspect; third, there is DNA with known attributes that is used as a control. All the DNA runs through the same tests at the same time. First, the three sets of DNA are purified and replicated, giving the scientists more DNA to work with. Next, the DNA is cut using a restriction enzyme. All three sets of DNA are then placed on the same block of gel. When electricity is passed through the gel, the DNA will migrate down the gel block; short strands will move farther down the gel than the long strands. This organizes the DNA strands by length so that the results can be accurately measured. A thin film of nylon is laid over the gel to blot up the DNA. Finally, short, radioactive strands of DNA called probes are sent onto the nylon. These radioactive probes will pair up with complementing strands of DNA on the nylon. The probes are designed to seek out and pair up with uncommon sequences of DNA; the probes will all congregate to the places where the uncommon sequences from the replicated DNA have come to rest.
When the nylon film is X-rayed, any large concentrations of the radioactive probes will leave blots on the X-ray print. If the blots from the control DNA look different than they normally do, the scientists know that there was error in the test, and the results are thrown out. If the control blots are in the correct positions, the test was run properly. If the blots from the suspect line up with the blots from the DNA gathered at the crime scene, then the two samples came from the same person, and the suspect was indeed present at the crime scene.
Many have questioned the validity of DNA fingerprinting test results. Because the burden of proof is on the forensic scientists, there are safeguards in place to be sure that DNA fingerprinting results are accurate. The first is the control DNA that runs through the tests alongside the suspect's DNA and the DNA found at the crime scene. If the control DNA tests properly, then the DNA in question tested properly as well. Another safeguard in place is that the test is run four times using four different restriction enzymes. In order to be absolutely positive of a match, all four sets of DNA from the crime scene must match all four sets of DNA from the suspect. With these safeguards in place, the odds are one in 70 billion (140 times Earth's population) that the results are inaccurate.
However, DNA fingerprinting is sometimes refutable evidence, even with such low odds of innacuracy. If the technician confused the DNA samples or deliberately gave false results, the information gleaned from a DNA test would be useless. Also, DNA evidence only proves that a person was present. It does not prove that the depositor of the DNA was the actual criminal. For these reasons, there must be other evidence to support the DNA fingerprints' conclusion; the fingerprints alone are not enough for a conviction.