It is the small difference in base pair sequences of DNA that make the phenotypic appearance of each individual unique. An easier and quicker solution to comparing DNA sequences is DNA fingerprinting. In human beings, ninety-nine per cent of DNA base sequences are identical and are known as the bulk genomic DNA. The remaining one per cent DNA base sequences differ and are present as a small stretch of repeated sequences known as repetitive DNA. DNA fingerprinting identifies the differences in this region. To separate both genomic as well as repetitive DNA the process of density gradient centrifugation is carried out. As satellite DNA is lighter and bulk DNA is heavier, so they get separated on the basis of their density. Graphical representation shows bulk genomic DNA as a major peak and repetitive DNA as smaller peaks known as satellite DNA.
Satellite DNA is highly repetitive and consists of non-coding sequences. Based on the length of the segment, base composition and number of repetitive units satellite DNA can be classified as mini-satellite DNA and micro-satellite DNA. Mini-satellite is a section of DNA which has a variable number of tandem repeats or VNTR. This step is followed by the hybridisation of the DNA fragments using a radio-labelled VNTR probe. Finally, the hybridised DNA fragments are detected by a technique called autoradiography conducted using an X-ray film. Hybridisation with the VNTR probe results in an autoradiogram, which produces several bands of different sizes. These bands provide a characteristic pattern to an individual’s DNA and vary from one individual to another except in identical or monozygotic twins. Today, the accuracy of the DNA fingerprinting technique has further improved due to the advent of the polymerase chain reaction or PCR, where multiple copies of a single DNA sequence can be made. DNA polymorphism is the guiding principle behind genetic mapping and therefore it helps in the DNA fingerprinting technique. The DNA fingerprinting technique was developed by Alec Jeffreys. DNA fingerprinting technique helps in crime investigation, paternity testing, determining genetic and population diversity and studying evolution and speciation.