dna finger printing

DNA fingerprinting 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.