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DNA Fingerprinting

DNA fingerprint of an individual is the Southern blot of his DNA that is digested with a restriction endonuclease and hybridized with a specific radioactive probe (generally DNA). This techhnique, for the first time, was developed by Alec Jeffreys (1985-86) and colleagues in U.K. and is extremely reliable in comparison to the conventional analysis techniques of serum proteins, erythrocyte antigens (A, B, O blood group), etc. It is used with almost absolute certainty for many purpose, e.g.,

        i.            Identification of criminals through blood stains, semen stains, hair roots, etc.,

ii.            Solution of dispute cases of parentage (earlier identified by blood groups that has many limitations)

iii.            Identification of disputed immigrants by DNA data bank that has been or is being prepared throughout the world, etc.

iv.            Matching of donor bone marrow cells with that of the bone cancer patient in bone marrow transplants.

v.            Detection of trait markers.

vi.            Identification of plant varieties and microorganisms for patents.

For DNA fingerprinting:

        i.            The DNA samples, usually from the blood of suspected person and his or her close relatives for comparison, are obtained. DNA is also extracted from the blood stains on clothings of the murder victim, the semen from vagina of rape victims, years old bone marrow of murder victims (as in Nithari case), etc. recovered from the evidence of crime, that can be amplified by PCR (Polymerase Chain Reaction) technique, if small in quantity.

ii.            These DNA samples are separately digested with a suitable restriction enzyme that cleaves the DNAs at specific sites to produce DNA fragments of various lengths.

iii.            The DNA fragments thus obtained are loaded in different wells along with the standards molecular weights or which arc known and the kits arc available in the market) in agarose gel slab and subjected to gel-electrophoresis.

    iv.            The DNA fragments of various bands are denatured (made single-stranded) and transferred onto a nitrocellulose membrane by Southern blotting. Thereafter, the DNAs are fixed by baking the nitrocellulose membrane at 80o C.

v.            The single-stranded DNA fragments are then hybridized with the appropriate radioactive proble (generally single-stranded DNA) that is usually prepared from mini-satellite or micro-satellite DNA (VNTR or variable number of tandem repeats), e.g., a universal probe, bkm (branded karait microsatellite DNA) probe, has been developed from the sex chromosome of banded karait that is useful in human beings, plats, etc.

vi.            After washing off or the free probe, the hybridized DNA fragments with the probe (due to complementary base pairing) are detected by autoradiography, where the nitrocellulose membrane is covered with a photographic (or X-ray) film in dark leading to its exposure at the locations of radioactive probe.

vii.            In DNA fingerprint, the bands of suspected criminal and that of the test DNA from the evidence of crime (blood stain, semen stain, etc.) thus obtained are compared. If the suspected person is involved in crime, the bands match perfectly. In case of disputed parentage, the DNA fingerprints of the child, the mother and the suspected father are compared where some of the band of child match with the bands of mother, whereas, rest with the father (Fig. 12.22 and 12.23).

The possibility of two persons having same DNA fingerprints is very rare.