DNA finger printing

DNA finger printing

(1) Alec Jeffreys et al (1985) developed the procedure of genetic analysis and forensic medicine, called DNA finger printing.

(2) It is individual specific DNA identification which is made possible by the finding that no two people are likely to have the same number of copies of repetitive DNA sequences of the regions.

(3) It is also known as DNA profiling.

(4) The chromosomes of every human cell contain scattered through their DNA short, highly repeated 15 nucleotide segments called “mini-satellites” or variable-number Tandem Repeat (VNTR).

Technique for DNA fingerprinting

(1) Only a small amount of tissues like blood or semen or skin cells or the hair root follicle is needed for DNA fingerprinting.

(2) Typically DNA content of about 100,000 cells or about 1 microgram is sufficient.

(3) The procedure of DNA fingerprinting involves the following major steps:

(i) DNA is isolated from the cells in a high-speed refrigerated centrifuge.

(ii) If the sample of DNA is very small, DNA can be amplified by Polymerase Chain Reaction (PCR).

(iii) DNA is then cut up into fragments of different length using restriction enzymes.

(iv) The fragments are separated according to size using gel electrophoresis through an agarose gel. The smaller fragments move faster down the gel than the larger ones.

(v) Double stranded DNA is then split into single stranded DNA using alkaline chemicals.

(vi) These separated DNA sequences are transferred to a nylon or nitrocellulose sheet placed over the gel. This is called ‘Southern Blotting’ (after Edward Southern, who first developed this method in 1975).

(vii) The nylon sheet is then immersed in a bath and probes or makers that are radioactive synthetic DNA segments of known sequences are added. The probes target a specific nucleotide sequence which is complementary to VNTR sequences and hybridizes them.

(viii) Finally, X-ray film is exposed to the nylon sheet containing radioactive probes. Dark bands develop at the probe sites which resemble the bar codes used by grocery store scanners to identify items.

Applications of DNA fingerprinting

This technique is now used to:

(i) Identify criminals in forensic laboratories.

(ii) Settle paternity disputes.

(iii) Verify whether a hopeful immigrant is, as he or she claims, really a close relative of already an established resident.

(iv) Identify racial groups to rewrite biological evolution.