Write an explanatory note on $DNA$ fingerprinting.

(N/A) $DNA$ fingerprinting is a quick method to analyze differences in the $DNA$ sequences of two individuals.
It involves the identification of differences in repetitive $DNA$.
Repetitive $DNA$ is a specific region in $DNA$ where a small stretch of $DNA$ is repeated many times.
Through density gradient centrifugation,these repetitive $DNA$ sequences 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 the human genome.
Satellite $DNA$ is classified into categories such as microsatellites and minisatellites based on the length of the segment,the number of repetitive units,and the base composition ($A:T$-rich or $G:C$-rich).
Satellite $DNA$ shows a high degree of polymorphism,which forms the basis of $DNA$ fingerprinting.
Polymorphism is the variation in individuals at the genetic level,arising 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,ranging from single nucleotide changes to large-scale changes.
In an individual,$DNA$ from every tissue (e.g.,blood,hair follicle,skin,bone,saliva) shows the same degree of polymorphism.
Thus,it becomes an essential identification tool in forensic applications.
As polymorphisms are inherited from parents to children,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 markers for inherited diseases.
He used satellite $DNA$ as a probe that shows a very high degree of polymorphism,called Variable Number of Tandem Repeats $(VNTRs)$.
The technique involves Southern blot hybridization using radiolabeled $VNTR$ as a probe.
The steps are:
$(i)$ $DNA$ isolation: $DNA$ is extracted from cells.
$(ii)$ Amplification: Many copies of the extracted $DNA$ can be made using the Polymerase Chain Reaction $(PCR)$.
$(iii)$ Digestion: Digestion of $DNA$ by restriction endonucleases.
$(iv)$ Separation: Separation of $DNA$ fragments by gel electrophoresis.
$(v)$ Southern Blotting: Transfer of separated $DNA$ fragments to synthetic membranes (like nylon or nitrocellulose).