Write a explanatory note on $DNA$  Fingerprinting
 

$DNA$ fingerprinting is a very quick way to analyse the differences in the $DNA$ sequences of two individuals.

It involves the identification of differences in repetitive $DNA.$

Repetitive $DNA$ is a specific region in $DNA$ in which a small stretch of $DNA$ is repeated many times.

Through density gradient centrifugation these repetitive $DNA$ are separated from the bulk genomic $DNA.$

Bulk $DNA$ forms a major peak during centrifugation and the other small peaks are known as satellite $DNA.$

These sequences do not code for any proteins normally, but they constitute a large portion of human genome.

The satellite $DNA$ is classified into many categories such as microsatellites, minisatellites etc. on the basis of length of segment, number of repetitive units the base composition ($A$ : $T$ - rich or $\mathrm{G}: \mathrm{C}$ - rich ) etc.

The satellite $DNA$ also show high degree of polymorphism and forms the basis of $DNA$ fingerprinting.

Polymorphism : It is the variation in individuals at genetic level.

Polymorphism arises due to mutations.

It plays an important role in evolution and speciation.

In a population, if an inheritable mutation is observed at high frequency, it is referred to as $DNA$ polymorphism.

There are different types of polymorphism from single nucleotide change to large scale changes.

In an individual, $DNA$ from every tissue (eg., blood, hair follicle, skin, bene, saliva etc.) shows the same degree of polymorphism.

- Thus, they become very essential identification tool in forensic applications.

- As polymorphisms are inherited from parents to children.

So it is useful in paternity testing.

- Technique of $DNA$ Fingerprinting : Alec Jeffreys initially developed $DNA$ fingerprinting also known as $DNA$ typing or $DNA$ profiling to find out markers for the inherited diseases.

He used a satellite $DNA$ as probe that shows very high degree of polymorphism and called it variable number of Tandem Repeats ($VNTRs$).

- The technique involved southern blot hybridisation using radiolabelled $VNTR$ as a probe.

The technique has the following steps :

$(i)$ $DNA$ isolations: $DNA$ is extracted from the cells in a high speed centrifuge.

$(ii)$ Amplification : Amplification many copies of the extracted $DNA$ can be made by the use of polymerase chain reaction.

$(iii)$ Digestion: Digestion of $DNA$ by restriction endonucleases.

$(iv)$ Separation : Separation of $DNA$ fragments by electrophoresis.

$(v)$ Southern Blotting : Transfer of separated $DNA$ fragments to synthetic membranes (like nylon or nitrocellulose).g