DNA Fingerprinting

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 You will be amazed to know that over 99.9%of base sequences among human is the same. Now,the 0.1% differnce in base sequence make every individual unique in their phenotypic appearance. DNA fingerprinting is the best method to find out genetic differences among the individual of a population. Sequencing DNA every time is very expensive and daunting task. 


DNA fingerprinting involves identifying differences in some specefic regions in DNA sequence, these are called repetitive DNA sequence,because in these sequences; a small stretch of DNA is repeated many times. These repetitive DNA are  seprated from bulk genomic DNA as differnt peaks during density gradient centrifugation. The bulk DNA forms major peaks and other small peaks are known as satellite DNA. Depending on base composition(A:T rich or G:C rich),length of segment and number of repetitive units, the satellite DNA is classified into many categories; such as micro satellite ,mini satellite etc. These sequences normally do not code for any protein, but they form a large portion of human genome. These sequences show High degree of polymorphism and form the basis of DNA fingerprinting. Since DNA from every tissue from an individual shows the same degree of polymorphism, they become very useful identification tool in forensic applications. As the polymorphism are inheritable from parents to children, DNA fingerprinting is the basis of paternity testing in case of child disputes. 


The technique of DNA fingerprinting was initially developed by Alec Jeffreys. He used a satellite DNA as probe that shows very high degree of polymorphism. It was called as  Variable Number of Tandem Repeats  (VNTR). The technique used earlier involved southern blot hybridisation using radiolabelled VNTR as a probe. 

It involves:

- isolation of DNA 

- digestion of DNA with restriction endonucleases 

- sepration of DNA fragments by gel electrophoresis 

- blotting (transferring)  of seprated DNA fragments to synthetic membranes; such as nitrocellulose or nylon.

- hybridisation using VNTR probe

- detection of hybridised DNA fragments by autoradiography 


VNTR belongs to the class of minisatellite DNA. A small DNA sequences is arranged tandemly in many copy numbers. The copy number vary from chromosome to chromosome in an individual. The numbers of repaeat shows very high degree of polymorphism. As a res ult the size of VNTR vary from 0.1to 20 kb. Consequently, after hybridisation with VNTR probe, the aitoradiogram gives many bands of differing sizes .These bands gives a charactrstic pattern for an individual DNA. This pattern differ from individual to individual in a population except in case of monozygotic (identical) twins. DNA from single cell is enough to perform DNA fingerprinting analysis. 

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