DNA fingerprinting

DNA fingerprinting or DNA profiling, any of several similar techniques for analyzing and comparing DNA from separate sources, used especially in law enforcement to identify suspects from hair, blood, semen, or other biological materials found at the scene of a violent crime. It depends on the fact that no two people, save identical twins, have exactly the same DNA sequence, and that although only limited segments of a person's DNA are scrutinized in the procedure, those segments will be statistically unique.

Methods

A common procedure for DNA fingerprinting is restriction fragment length polymorphism (RFLP). In this method, DNA is extracted from a sample and cut into segments using special restriction enzymes. RFLP focuses on segments that contain sequences of repeated DNA bases, which vary widely from person to person. The segments are separated using a laboratory technique called electrophoresis, which sorts the fragments by length. The segments are radioactively tagged to produce a visual pattern known as an autoradiograph, or "DNA fingerprint," on X-ray film. A newer method known as short tandem repeats (STR) analyzes DNA segments for the number of repeats at 13 specific DNA sites. The chance of misidentification in this procedure is one in several billion. Yet another process, polymerase chain reaction , is used to produce multiple copies of segments from a very limited amount of DNA (as little as 50 molecules), enabling a DNA fingerprint to be made from a single hair. Once a sufficient sample has been produced, the pattern of the alleles (see genetics ) from a limited number of genes is compared with the pattern from the reference sample. A nonmatch is conclusive, but the technique provides less certainty when a match occurs.

Applications

In criminal investigations, the DNA fingerprint of a suspect's blood or other body material is compared to that of the evidence from the crime scene to see how closely they match. The technique can also be used to establish paternity. First developed in the mid-1980s, DNA fingerprinting has been accepted in most courts in the United States, and has in several notable instances been used to exonerate or free persons convicted of crimes. All states have established DNA fingerprint databases, and the Federal Bureau of Investigation has instituted a national DNA fingerprint database linking those of the states and including DNA collected in connection with federal offenses. DNA fingerprinting is generally regarded as a reliable forensic tool when properly done, but some scientists have called for wider sampling of human DNA to insure that the segments analyzed are indeed highly variable for all ethnic and racial groups.

The techniques used in DNA fingerprinting also have applications in paleontology, archaeology, various fields of biology, and medical diagnostics. It has, for example, been used to match the goatskin fragments of the Dead Sea Scrolls. In biological classification , it can help to show evolutionary change and relationships on the molecular level, and it has the advantage of being able to be used even when only very small samples, such as tiny pieces of preserved tissue from extinct animals, are available.

DNA fingerprinting also known as DNA typing or genetic fingerprinting, is a method for identifying individuals by the particular structure of their DNA. It gained its name because the structure of the DNA of each person is different, and hence, just as each of us is unique with respect to the pattern of our fingerprints, so we can be identified from our DNA.

As well as containing the 100 000 or so genes that encode the structure of the thousands of proteins from which human beings are constructed, there are large regions of our DNA that do not consist of genes and appear to serve no useful purpose. Part of this functionless, ‘junk’ DNA is made up of long stretches of repeated sequences of the four nucleotide building blocks from which DNA is constructed. There is, however, some order in these repeats. For example, they may form what are called hypervariable regions, also known as mini-satellite DNA, which consist of blocks of tandem repeats of a short ‘core’ sequence. Nearly 100 of these hypervariable regions have been found in the human genome, many but not all of which are close to genes that encode different proteins. The number of copies in these different families of repeats varies widely between unrelated people and thus constitutes a unique genetic profile, or fingerprint. They are of particular value because they are apparently dispersed randomly throughout the genome and therefore are inherited independently of each other.

To produce a DNA fingerprint, DNA from a cell sample is digested with enzymes that cut it up into many different sized pieces and the mixture is placed in a gel. This is then exposed to an electric field and the fragments migrate to different positions by virtue of their size. In this way a pattern is obtained that reflects different numbers of repeats in different individuals; the length of a particular DNA fragment is a function of the number of repeats present.

After the separation of the fragments is complete, the DNA is transferred to a nitrocellulose filter, on which it is immobilized. The position of the fragments containing the repeats is identified by the use of a radioactively labelled DNA probe designed to bind to the core repeat sequences. The fingerprint is visualized by placing an X-ray plate over the filter and developing the film. Since mini-satellite DNA has a relatively high mutation rate, and this varies between different hypervariable regions, in practice it is important to ensure that the rates of mutation of the mini-satellites used for testing are not too great, so as to avoid false exclusions.

DNA fingerprinting is used for many purposes, particularly paternity testing and for forensic work. Of particular concern to the criminal fraternity is that DNA for fingerprinting can be obtained from whole blood, semen, vaginal fluid, hair roots, almost any tissue, and even from bones that have been buried for a long time. The probability that two unrelated individuals show exactly the same pattern varies depending on the particular hypervariable regions that are chosen. In one commonly used system the region analysed yields up to 36 different sized DNA bands, or alleles, for each individual. A band-sharing statistic is estimated at 0.25; that is, the probability of two unrelated individuals sharing the same pattern is 0.2536³⁶ or one in 5000 billion billion!

Because of its extreme sensitivity, and because appropriate hypervariable regions can be amplified from minute traces of DNA to produce diagnostic patterns, this technique has revolutionized forensic medicine over recent years.

D. J. Weatherall

Bibliography

Gill, P. (1994). DNA typing. In The enclycopaedia of molecular biology, pp. 286–8. Blackwell Science, Oxford.
Jeffreys, A. J. et al. (1986). DNA fingerprinting and segregation analysis of multiple markers in human pedigrees. American Journal of Human Genetics, 39, 11–24.

DNA fingerprinting (genetic fingerprinting) A technique in which an individual's DNA is analysed to reveal the pattern of repetition of particular short nucleotide sequences (called variable number tandem repeats) throughout the genome. This pattern is claimed to be unique to the individual concerned, and the technique is therefore used for identification purposes in forensic science and paternity disputes, and in veterinary science. Sufficient DNA can be obtained from very small samples of body tissue, such as blood, semen, or hair, if necessary by using the polymerase chain reaction to amplify minute quantities of DNA. Restriction enzymes are used to cleave the DNA and the marker sequences are revealed by specific gene probes and Southern blotting. See also microsatellite DNA; randomly amplified polymorphic DNA.