DNA Fingerprinting Technique

Every human being shares essentially the same basic chemical structure of DNA, yet there are literally millions of small differences scattered across the sequence of base pairs between any two individuals, and it is precisely this sequence variation that makes each person, other than identical twins, genetically distinguishable from everyone else -- in much the same way that each person's actual fingerprints are unique. The technique that exploits this variation to identify individuals from their DNA, DNA fingerprinting, was first developed by the British geneticist Alec Jeffreys in 1985 (later recognised with the Royal Society's Copley Medal in 2014). Human DNA is organised into 23 pairs of chromosomes carrying roughly 1.5 million pairs of genes in total, and not every stretch of this DNA codes for protein: alongside the coding regions, some segments perform regulatory functions, some are non-coding intervening sequences (introns), and still others consist of short sequences repeated many times over, called repetitive DNA. When bulk genomic DNA is separated by density-gradient centrifugation, this repetitive DNA forms distinct minor peaks alongside the single large peak formed by the bulk of ordinary genomic DNA, and these minor peaks are collectively referred to as satellite DNA; depending on their base composition (whether they are relatively A:T-rich or G:C-rich), their overall length and how many times the repeat unit recurs, satellite DNA sequences are further classified into several sub-categories, including microsatellites and minisatellites. These sequences carry no direct protein-coding function of their own, yet they make up a substantial fraction of the total human genome, and because they display an unusually high degree of variability (polymorphism) between different individuals, they form the molecular basis of DNA fingerprinting. The particular class of repetitive DNA sequence exploited for this purpose, whose exact copy number at a given locus differs meaningfully from one person to another, is called a Variable Number Tandem Repeat (VNTR), and it is variation in these VNTR copy numbers between individuals that DNA fingerprinting technique is specifically designed to detect and visualise. The overall procedure runs through several distinct stages: DNA is first extracted from a biological sample, which might be blood, semen, vaginal fluid, hair roots, teeth or bone; if only a very small quantity of DNA is available, it can first be amplified into many identical copies using the Polymerase Chain Reaction (PCR); the amplified DNA is then cut into smaller fragments at specific recognition sites using restriction enzymes; these fragments are separated by size through agarose gel electrophoresis, which sorts DNA fragments into bands based on how far they migrate through the gel; the separated DNA within the gel is then denatured, converted into single strands, using either alkaline chemicals or heat; the resulting band pattern is transferred out of the fragile gel and onto a more durable nylon membrane through a technique called Southern blotting; a radioactive (or, alternatively, fluorescent) DNA probe, itself complementary in sequence to the VNTR region being examined, is then added and allowed to hybridise, or base-pair, specifically with its matching restriction fragments on the membrane; any excess, unbound probe is washed away; and finally, an X-ray film placed against the membrane is exposed by the bound radioactive probe and then developed, producing a visible pattern of thick and thin dark bands, a pattern of bars that together constitute that individual's unique genetic fingerprint. DNA fingerprinting has proved valuable across a wide range of practical applications: in forensic analys …