Write a explanatory note on $DNA$ Fingerprinting
 

$DNA$ fingerprinting is a very quick way to compare the $DNA$ sequences of any two individuals.
          $DNA$ fingerprinting involves identifying differences in some specific regions in $DNA$ sequence called as repetitive $DNA$, because in these sequences, a small stretch of $DNA$ is repeated many times.

These repetitive $DNA$ are separated from bulk genomic $DNA$ as different peaks during density gradient centrifugation. The bulk $DNA$ forms a major peak and the other small peaks are referred to as satellite $DNA$ .

Depending on base composition ($A:T$ rich or $G:C$ rich), length of segment, and number of repetitive units,

These sequences normally do not code for any proteins, but they form a large portion of human genome. These sequence show high degree of polymorphism and form the basis of $DNA$ fingerprinting. 

          As polymorphism in $DNA$ sequence is the basis of genetic mapping of human genome as well as of $DNA$ fingerprinting, it is essential that we understand what $DNA$ polymorphism means in simple terms. Polymorphism (variation at genetic level) arises due to mutations.  New mutations may arise in an individual either in somatic cells or in the germ cells (cells that generate gametes in sexually reproducing organisms). If a germ cell mutation does not seriously impair individual’s ability to have offspring who can transmit the mutation, it can spread to the other members of population (through sexual reproduction).

Allelic (again recall the definition of alleles from Chapter $5$) sequence variation has traditionally been described as a $DNA$ polymorphism if more than one variant (allele) at a locus occurs in human population with a frequency greater than $0.01$ . In simple terms, if an inheritable mutation is observed in a population at high frequency, it is referred to as $DNA$​ polymorphism. The probability of such variation to be observed in noncoding $DNA$ sequence would be higher as mutations in these sequences may not have any immediate effect/impact in an individual’s reproductive ability.

These mutations keep on accumulating generation after generation, and form one of the basis of variability/polymorphism. There is a variety of different types of polymorphisms ranging from single nucleotide change to very large scale changes. For evolution and speciation, such polymorphisms play very important role, and you will study these in details at higher classes.
          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, as used earlier, involved Southern blot hybridisation using radiolabelled $VNTR$ as a probe. It included
$(i)$ isolation of $DNA$,
$(ii)$ digestion of $DNA$ by restriction endonucleases,
$(iii)$ separation of $DNA$  fragments by electrophoresis,
$(iv)$ transferring (blotting) of separated $DNA$ fragments to synthetic membranes, such as nitrocellulose or nylon,
$(v)$ hybridisation using labelled $VNTR$ probe, and
$(vi)$ detection of hybridised $DNA$ fragments by autoradiography.