DNA fingerprinting is the term applied to a range of techniques that are used to show similarities and dissimilarities between the DNA present in different individuals.
DNA fingerprinting is an important tool in the arsenal of forensic investigators and intelligence officers. In an era when plastic surgery can be used to alter a terrorist's appearance, DNA fingerprinting allows for positive identification not only of body remains, but also of suspects in custody. DNA fingerprinting can also link physical evidence from incidents that occur in different parts of the world.
Sir Alec Jeffreys at the University of Leicester developed DNA fingerprinting in the mid 1980s. The sequence of nucleotides in DNA is similar to a fingerprint, in that it is unique to each person. DNA fingerprinting is used for identifying people, studying populations, and forensic investigations.
Historical Uses of DNA Fingerprinting
Jeffreys was first given the opportunity to demonstrate the power of DNA fingerprinting in March of 1985 when he proved a boy was the son of a British citizen and should be allowed to enter the country. In 1986, DNA was first used in forensics. In a village near Jeffreys' home, a teenage girl was assaulted and strangled. No suspect was found, although body fluids were recovered at the crime scene. When another girl was strangled in the same way, a 19-year-old caterer confessed to one murder but not the other. DNA analysis showed that the same person committed both murders, and the caterer had falsely confessed. Blood samples of 4582 village men were taken, and eventually the killer was revealed when he attempted to bribe someone to take the test for him.
The first case to be tried in the United States using DNA fingerprinting evidence was of African-American Tommie Lee Edwards. In November 1987, a judge did not permit population genetics statistics that compared Edwards to a representative population. The judge feared the jury would be overwhelmed by the technical information. The trial ended in a mistrial. Three months later, Andrews was on trial for the assault of another woman. This time the judge did permit the evidence of population genetics statistics. The prosecutor showed that the probability that Edwards' DNA would not match the crime evidence was one in ten billion. Edwards was convicted.
DNA fingerprinting has been used repeatedly to identify human remains. In Cardiff, Wales, skeletal remains of a young woman were found, and a medical artist was able to make a model of the girl's face. She was recognized by a social worker as a local run-away. Comparing the DNA of the femur of the girl with samples from the presumptive parents, Jeffreys declared a match between the identified girl and her parents. In Brazil, Wolfgang Gerhard, who had drowned in a boating accident, was accused of being the notorious Nazi of Auschwitz, Josef Mengele. Disinterring the bones, Jeffreys and his team used DNA fingerprinting to conclude that the man actually was the missing Mengele.
In addition to forensics, DNA has been used to unite families. In 1976, a military junta in a South American country killed over 9000 people, and the orphaned children were given to military couples. After the regime was overthrown in 1983, Las Abuelas (The Grandmothers) determined to bring these children to their biological families. Using DNA fingerprinting, they found the families of over 200 children.
DNA has been used to solve several historical mysteries. On July 16, 1918, the czar of Russia and his family were shot, doused with sulfuric acid, and buried in a mass grave. In 1989, the site of burial was uncovered, and bone fragments of nine skeletons were assembled. DNA fingerprinting experts from all over the world pieced together the puzzle that ended in a proper burial to the Romanov royal family in Saint Petersburg in 1998.
The Mechanics of DNA Fingerprinting
The nucleus of every cell in the human body contains deoxyribonucleic acid or DNA, a biochemical molecule that is made up of nearly three-billion nucleotides. DNA consists of four different nucleotides, adenine (A), thymine (T), guanine (G), and cytosine (C), which are strung together in a sequence that is unique to every individual. The sequence of A, T, G, and C in human DNA can be found in more combinations or variations than there are humans. The technology of DNA fingerprinting is based on the assumption that no two people have the same DNA sequence.
The DNA from a small sample of human tissue can be extracted using biochemical techniques. Then the DNA can be digested using a series of enzymes known as restriction enzymes, or restriction endonucleases. These molecules can be thought of as chemical scissors, which cut the DNA into pieces. Different endonucleases cut DNA at different parts of the nucleotide sequence. For example, the endonuclease called SmaI cuts the sequence of nucleotides CCCGGG between the third cytosine (C) and the first guanine (G).
After being exposed to a group of different restriction enzymes, the digested DNA undergoes gel electrophoresis. In this biochemical analysis technique, test samples of digested DNA are placed in individual lanes on a sheet of an agarose gel that is made from seaweed. A separate lane contains control samples of DNA of known lengths. The loaded gel is then placed in a liquid bath and an electric current is passed through the system. The various fragments of DNA are of different sizes and different electrical charges. The pieces move according to their size and charge with the smaller and more polar ones traveling faster. As a result, the fragments migrate down the gel at different rates.
After a given amount of time, the electrical current in the gel electrophoresis instrumentation is shut off. The gel is removed from the bath and the DNA is blotted onto a piece of nitrocellulose paper. The DNA is then visualized by the application of radioactive probe that can be picked up on a piece of x-ray film. The result is a film that contains a series of lines showing where the fragments of DNA have migrated. Fragments of the same size in different lanes indicate the DNA has been broken into segments of the same size. This demonstrates a similarity between the sequences under test.
Different enzymes produce different banding patterns and normally several different endonucleases are used in conjunction to produce a high definition banding pattern on the gel. The greater the number of enzymes used in the digestion, the finer the resultant resolution.
In DNA fingerprinting, scientists focus on segments of DNA in which nucleotide sequences vary a great deal from one individual to another. For example, five to ten percent of the DNA molecule contains regions that repeat the same nucleotide sequence many times, although the number of repeats varies from person to person. Jeffreys targeted these long repeats called variable number of tandem repeats (VNTRs) when he first developed DNA fingerprinting. The DNA of each person also has different restriction fragment sizes, called restriction fragment length polymorphisms (RFLPs), which can be used as markers of differences in DNA sequences between people. Today, technicians also use short tandem repeats (STRs) for DNA fingerprinting. STRs are analyzed using polymerase chain reaction or PCR, a technique for mass-producing sequences of DNA. PCR allows scientists to work with degraded DNA.
Use as a forensic tool. DNA fingerprinting is now an important tool in the arsenal of forensic chemists. It is used in forensics to examine DNA samples taken from a crime scene and compare them to those of a suspect. Criminals almost always leave evidence of their identity that contains DNA at the crime scene—hair, blood, semen, or saliva. These materials can be carefully collected from the crime scene and fingerprinted
Although DNA fingerprinting is scientifically sound, the use of DNA fingerprinting in courtrooms remains controversial. There are several objections to its use. Lawyers who misrepresent the results of DNA fingerprints may confuse jurors. DNA fingerprinting relies on the probability that individuals will not produce the same banding pattern on a gel after their DNA has been fingerprinted. Establishing this probability relies on population statistics. Each digested fragment of DNA is given a probability value. The value is determined by a formula relating the combination of sequences occurring in the population. There is concern that not enough is known about the distribution of banding patterns of DNA in the population to express this formula correctly. Concerns also exist regarding the data collection and laboratory procedure associated with DNA fingerprinting procedures. For example, it is possible that cells from a laboratory technician could be inadvertently amplified and run on the gel. However, because each person has a unique DNA sequence and this sequence cannot be altered by surgery or physical manipulation, DNA fingerprinting is an important tool for solving criminal cases.
█ FURTHER READING:
Griffiths, A., et al. Introduction to Genetic Analysis, 7th ed. New York: W.H. Freeman and Co., 2000.
Jorde, L. B., J. C. Carey, M. J. Bamshad, and R. L. White. Medical Genetics, 2nd ed. Mosby-Year Book, Inc., 2000.
Klug, W., and M. Cummings. Concepts of Genetics, 6th ed. Upper Saddle River: Prentice Hall, 2000.
Watson, J. D., et al. Molecular Biology of the Gene, 4th ed. Menlo Park, CA: The Benjamin/Cummings Publishing Company, Inc., 1987.
The University of Washington. "Basics of DNA fingerprinting." < http://www.biology.washington.edu/fingerprint/dnaintro.html ,>(March 4, 2003).