DNA fingerprinting is a method for comparing the DNA sequences of any two individuals. Repetitive DNA is separated from bulk genomic DNA since it appears as a distinct peak during density gradient centrifugation. Satellites are of two types—micro-satellites and mini satellites, depending upon the base composition, length of segment and the number of repetitive units.
Satellites do not code for proteins, but have a major role to play in DNA fingerprinting.
Polymorphism is actually a result of mutation. A germ cell mutation (which can pass on to the next generation through sexual reproduction) gives rise to polymorphism in populations.
In other words, an inheritable mutation if observed in higher frequencies in a population is known as polymorphism. Polymorphisms arise normally in non-coding sequences because mutations in non-coding sequences do not affect an individual’s reproductive ability.
VNTR ( variable number of tandem repeats) are satellite DNAs that show high degree of polymorphism. VNTRs are used as probes in DNA fingerprinting.
First of all, DNA from an individual is isolated and cut with restriction endonucleases. Fragments are separated according to their size and molecular weight on gel electrophoresis. Fragments separated on electrophoresis gel are blotted (immobilised) on a synthetic membrane such as nylon or nitrocellulose. Immobilised fragments are hybridised with a VNTR probe. Hybridised DNA fragments can be detected by autoradiography. VNTRs vary in size from 0.1 to 20 kb. Hence, in the autoradiogram, band of different sizes will be obtained. These bands are characteristic for an individual. They are different in each individual, except identical twins.
