DNA Fingerprinting

DNA fingerprinting is a molecular biology technique that identifies and compares specific, highly variable regions of an individual's DNA, producing a distinctive banding pattern unique (or very nearly unique) to that individual, which can then be used to establish identity or biological relationships with a very high degree of confidence — much as a traditional fingerprint uniquely identifies a person by the distinctive pattern of ridges on their fingertip. The technique was originally developed by the British geneticist Alec Jeffreys in 1984, and in India it was pioneered and further developed by Dr. Lalji Singh and his colleagues at the Centre for Cellular and Molecular Biology (CCMB), Hyderabad.

DNA fingerprinting exploits a category of DNA sequence called repetitive DNA: short nucleotide sequences that are repeated many times, one after another, at specific locations scattered throughout the genome. Two important classes of repetitive DNA are used for this purpose. Variable Number Tandem Repeats (VNTRs), also historically called "minisatellites," are relatively short DNA sequences (typically some tens of base pairs long) repeated a variable number of times in a row at a particular chromosomal location; Short Tandem Repeats (STRs), or "microsatellites," are similar but built from an even shorter core repeat unit (commonly just 2 to 6 base pairs long). The specific NUMBER of times such a repeat unit is repeated at a given chromosomal location varies considerably from one individual to another within a population — this person-to-person variability in repeat number is called polymorphism, and it is precisely this high degree of polymorphism at VNTR/STR loci (in sharp contrast to the majority of the genome, which is highly similar between different individuals) that makes DNA fingerprinting such a powerful method for individual identification.

Because each parent transmits exactly one copy of each chromosome (and hence one copy of each VNTR/STR locus on it) to each of their offspring, an individual's own specific set of VNTR/STR repeat-number values at several different chromosomal locations is inherited in a simple Mendelian fashion from their two biological parents — which is exactly why DNA fingerprinting can be used not only to positively identify a specific individual, but also to establish or rule out biological parentage and other close genetic relationships, since a true biological child's DNA fingerprint pattern must be explicable as a specific combination of alleles drawn from its two actual parents' own patterns.

The basic laboratory procedure for generating a DNA fingerprint involves several sequential steps: DNA is first extracted from a biological sample (which can be blood, hair, semen, or essentially any other tissue containing nucleated cells); the extracted DNA is then cut into fragments using specific restriction endonuclease enzymes; the resulting DNA fragments are separated by size using gel electrophoresis, in which an electric field drives the negatively charged DNA fragments through a porous gel matrix, with smaller fragments migrating faster (and hence farther) through the gel than larger fragments; the separated DNA fragments are then transferred (blotted) from the gel onto a nylon or nitrocellulose membrane (a technique called Southern blotting); and finally, the VNTR/STR fragments of interest are specifically visualised on the membrane using a radioactively or otherwise labelled DNA probe (called a VNTR probe) that hybridises specifically to the repetitive sequence being examined, producing the final, individual-specific autoradiographic banding pattern — the "DNA fingerprint" itself. …