DNA Fingerprinting


An individual's entire DNA sequence is composed of over 3 billion nucleotides,. Although the sequence is about 99.9% the same as other humans, the remaining 0.1% about 3 million nucleotides is unique to that individual. As a result, with the exception of identical twins, who have identical DNA sequences, no human ever has exactly the same DNA as another one. This genetic distinctiveness can be a powerful tool in identifying persons, in much the same way as a person's unique fingerprint.

Because nearly every cell in a person's body contains the same complete set of DNA, DNA isolated from dried blood, semen, or even a hair found at a crime scene can be compared to a DNA sample collected from a suspect and can prove whether he or she was present at the crime scene in much the same way as a person's normal fingerprints. Guy Paul Morin of Ontario, Canada, who was wrongfully convicted of murder, was exonerated through DNA testing over 10 years later.Using a procedure which quickly makes copies of DNA called the Polymerase Chain Reaction (or PCR), DNA analysis can be performed on minute amounts of tissue. This makes it harder for a criminal to remove all the evidence from the scene of a crime.

DNA fingerprinting can also be used in paternity testing to identify a child's biological parents, or in the wake of major accidents, disasters, or wars to identify those who have died. This powerful technology is not only useful in humans. Methods are currently being developed by the Canadian Forest Service in British Columbia to "fingerprint" valuable trees so that they can be traced if they are stolen.

How is it Done?

Theoretically, DNA fingerprinting could be carried out by sequencing entire genomes and comparing them to see if they match. But determining the exact sequence of the 3 billion nucleotides that make up a person's DNA would take years, and would be prohibitively expensive. Luckily, the variation between people is concentrated in particular regions of their DNA. These regions, which are short, highly-repeated 15-nucleotide segments, are called minisatellites. The locations and number of repeats of any particular minisatellite are highly variable. The probability that two unrelated individuals will have the same pattern of location and number of repeats in one minisatellite is about 1 in 10 billion. When several minisatellites are analysed at once, the probability diminishes even further and is for all intents and purposes zero.

So all we need to do is to locate a few of these minisatellites and find out how long they are. Their length gives an indication of how many times the 15 nucleotide sequence is repeated.

First, the DNA is chopped up using enzymes called restriction endonucleases, which cut the DNA at distinctive sites. Because the location of these sites varies from person to person, so will the lengths of the resulting fragments. In the illustration below, the enzyme used is called Eco R1, which recognizes the sequence: GAATTC (its complementary sequence: CTTAAG, is just GAATTC backwards!) and cuts between G and the first A.

The fragments are then sorted according to size and incubated with DNA Probes . The probes have been designed to bind to particular minisatellites. Usually, 5 to 10 types of probes which recognize different minisatellites are used simultaneously. When a probe binds to a DNA fragment, it makes that DNA detectable.

The detected fragments from two samples are compared. If the fragments are all the same size, the two samples can be concluded to have originated from the same individual. Finally, computers are used to calculate the probability that such a matching pattern could have occurred by coincidence. This calculation can be very complicated, as the frequency of different DNA patterns of different genes vary depending on the population. Usually, the chances of such a coincidence are several billion to 1, making DNA fingerprinting an extremely reliable method of identifying persons.