This is also known as ‘DNA PROFILING’ o ‘DNA TYPING’. DNA fingerprinting is a technique to identify a person on the basis of his/her DNA specificity.
The practice of using thumbs impression of a person, as an identifying mark is very well known since long.
The study of finger, palm and sole prints is called dermatoglyphics and it has been a subject of human interest.
But, the concept of DNA fingerprinting is totally a new approach in the field of molecular biology. Sir Alec Jeffreys (1985-86) invented the DNA fingerprinting technique at Leicester University, United Kingdom.
Meaning:
DNA of an individual carries some specific sequence of bases, which do not carry any information for protein synthesis. Such nucleotide base sequences are repeated many times and are found in many places throughout the length of DNA. The number of repeats is very specific in each individual. The tandem repeats of short sequences are called ‘mini satellites’ or ‘variable number tandem repeats’ (VNTRs). Such repeats are used as genetic markers in personal identity.
Technique:
1. The first step is to obtain DNA sample of the individual in question.
2. DNA is also isolated from bloodstains, semen stains or hair root from the body of the victim or from victim’s cloth even after many hours of any criminal offence. Even it can be obtained from vaginal swabs of rape victims. The amount of DNA needed for developing fingerprints is very small, only a few nanograms.
3. The DNA is digested with a suitable restriction endonuclease enzyme, which cuts them into fragments.
4. The fragments are subjected to gel electrophoresis by which the fragments are separated according to their size.
5. The separated fragments are copied onto a nitrocellulose filter membrane by Southern blotting technique.
6. Special DNA probes are prepared in the laboratory and made radioactive by labeling with radioactive isotopes. These probes contain repeated sequences of bases complimentary to those on mini satellites.
7. The DNA on the nitrocellulose filter membrane is hybridized with the radioactive probes and the free probes are washed off.
8. The bands to which the radioactive probes have been hybridized are detected through autoradiography. This is a technique where an X-ray film is exposed to the nitrocellulose membrane to mark the places where the radioactive DNA probes have bound to the DNA fragments. These places are marked as dark bands when X-ray film is exposed.
9. The dark bands on the X-ray film represent the DNA fingerprints or DNA profiles.
10. Comparison is made between the banding pattern of collected DNA sample and suspected human subject to confirm the criminal with hundred percent accuracy (Fig.5.24).
Significance:
1. The technique is extensively used as confirmatory test in crime detection in cases of rape and murder.
2. Disputed parentage can be solved by the technique.
3. This method can confirm species of more closeness or far apart from evolutionary point of view so that taxonomical problems can be solved.
4. The technique also can be used to study the breeding pattern of endangered animals.
5. Clinically this method can be used in restoring the health of blood cancer patients.