DNA Fingerprinting

By Michelle Tetreault, Ph.D.

The recent press coverage of the O.J. Simpson case has sparked a lot of interest in DNA fingerprinting. The concept is very similar to traditional fingerprinting. Just like each of us has unique patterns on our fingertips, we also have regions on our DNA that are very individualized. These regions, called "variable regions," are altered or rearranged frequently in the human population.

How likely is it for two people chosen at random to have an identical DNA fingerprint? A conservative estimate puts the chance at about one in 1 million. Even for two people of the same ethnic background the chance is less than one in 10,000 of them having the same DNA fingerprint.

The advantage to using DNA rather than traditional fingerprinting is you don't have to be as lucky. After all, an ordinary fingerprint can easily get smudged or masked altogether by gloves. But your DNA sequence is encoded into every cell of your body. All that is needed for analysis is a few cells from which the person's DNA can be extracted. This could come from a drop of blood, a semen sample, skin fragments from underneath a victim's nails, or even a few hairs (if the follicles are attached). Violent criminals often leave this kind of evidence behind.

The technique to produce DNA fingerprints is actually quite straight forward. The person's DNA is first cut up into small pieces at specific locations in the sequence. Since the positions and numbers of the variable regions differ from person to person, those DNA fragments that contain variable regions will be

different sizes for different people. If one could sort these pieces by size and stand them up in a row one would see a pattern that was specific to the person. If we had enough of these DNA fragments then any two people chosen at random would be very unlikely to have the same pattern.

The trick is then to sort the DNA by size and then identify which fragments have the variable regions. To do the sorting we make use of a convenient property of DNA–it is negatively charged! This means if we put DNA in a medium, and apply a voltage across it, the DNA feels the electric field and will be pulled through the medium. If the medium is viscous, the smaller pieces will feel less drag and so travel faster than the larger pieces. The pieces will be sorted by size.

In practice the DNA is placed in a horizontal line in a medium with an electrode directly below it and one some inches above it. As the DNA fragments move towards the upper electrode they separate to form distinct lines in the medium. If you add a chemical, like ethidium bromide, that both binds to DNA and fluoresces under ultraviolet light you can then see these lines with the aid of a W lamp. This technique is called "electrophoresis. "

But these lines are not yet the DNA fingerprint because we don't yet know which fragments contain the variable regions. In order to locate them, a special molecular probe must be used. This probe is typically itself a piece of DNA that has been made radioactive and will "stick" to these variable regions and to no others. Through a technique called a "Southern blot," these probes are attached to the variable regions. If we then expose the film to the blot, the now radioactive variable regions will show up as lines on the film. This at last is the DNA "fingerprint."

There are several variations of this method that will produce a DNA fingerprint. A new technology called Polymerase Chain Reaction (PCR), which I will discuss in my next column, has made it possible to quickly produce multiple copies of a DNA molecule. Rapid protocols have been developed to amplify only the variable regions in our DNA in such a machine.

Instead of running all of the DNA on the electrophoreses gel as in the above method, PCR enables technicians to run only the amplified variable regions which we are interested in. After staining with ethidium bromide, the DNA variable regions will fluoresce under ultraviolet light and can be photographed. This method then has no need for probing with radioactivity.

DNA fingerprinting is not only used in criminal cases. It can also be used to show parentage. Unlike traditional fingerprints, your DNA fingerprint is a combination of your mother and fathers' DNA fingerprint. The first legal case (1985) involving DNA made use of this fact to prove a family relationship so that a Ghanian boy could immigrate to England to join his mother and siblings. It can also be used to identify bodies in the case of a catastrophic accident like a plane crash.

Regardless of the present controversies, DNA fingerprinting will probably become common evidence for cases in the legal system. Constant improvements are being made in the technique. It won't be long until DNA analysis is as accepted in the courtroom as blood tests. Ultimately, DNA promises to be a powerful force for justice.

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