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Home » Biology Homework Help » Biotechnology » DNA Fingerprinting
DNA Fingerprinting
DNA fingerprinting or DNA profiling is generally used for the identification of criminals from blood strains, semen etc. and for establishing parentage in cases of dispute. The data from this approach are extremely reliable as compared to the conventional analysis of serum proteins and erythrocyte antigens and proteins. DNA fingerprint of an individual is essentially a Southern blot of his DNA digested with an endonucleases and probed with a radioactive DNA probe. The procedure for DNA fingerprinting is briefly described below:

1. The first step is to obtain a sample of DNA of the individual in question. The DNA is usually obtained from blood samples, but it may be isolated from several years old blood or semen stains on clothings, from vaginal swabs taken from rape victims, even from bone marrow of years old bones or murder victims etc. The amount of DNA needed for developing fingerprints is very small (a few nanograms).

2. The DNA is digested with a suitable restriction endonucleases, and the digest is subjected to get electrophoresis (Southern Hybridization).

3. The DNA is denatured and then transferred from the gel onto the nitrocellulose filter membrane, where it is fixed by backing the filter at 80˚C.

4. The DNA on the filter is hybridized with the appropriate radioactive probe (single-stranded DNA probe), the free probe is washed off and the bands to which the probe has hybridized are detected through autoradiography.

The probes used for DNA fingerprinting are usually prepared from mini-satellite or micro-satellite DNA. The mini-satellite DNA is highly variable mainly due to variations in the number of tandem repeats of a short core sequence, e.g. GGAGGTGGGCAGGAG (or A)G. Some examples of such probes are: probe 33.6, [(AGGGCTGGAGG)3]18; Probe 33.15 (AGAGGTGGGCAGGTGG)29; and Probe 33.5 (GGGA (or C) GTGGGCAGGAGG)14. These probes hybridize, under conditions of low stringency, to a number of the polymorphic loci represented by mini-satellite DNA. A simple universal probe, a tanden repeat of GATA, has been developed from the sex chromosomes of banded karait; this probe is reportedly very useful in DNA fingerprinting of not only man but of many other organisms, including plants.

The bands appearing in DNA fingerprint of the suspected criminal(s) are compared with those obtained from the test DNA recovered from the evidence for crime and believed to originate from the criminal, e.g. semen stains or sperm nuclei in vaginal swabs from rape victims, etc. there should be a perfect match in case the suspect had been involved. Ordinarily a molecular weight marker lane is run, and some related and unrelated control samples are also included.

In case of disputed parentage, the DNA fingerprints of the child, the mother and the suspected father are compared. As a rule, the bands present in a child’s fingerprint must be accounted for by those in the mother’s and the father’s fingerprints. Therefore, the child’s fingerprint is compared with that of the mother and their common bands are marked. The remaining bands in the child’s fingerprint must be present in that of child’s father.

In medicine, DNA fingerprinting has application in genetic counseling, tracing the frequency of donor cells in bone marrow  transplants, tissue culture cell line identification etc. In case of animal husbandry, proofs of parentage, poaching, identification in cases of theft or loss, detection of trait markers are the areas of application. Similarly, identification of plant varieties for patent, parentage and trait marker purposes, and of micro-organism strains is unequivocal with DNA fingerprinting.

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