DNA fingerprinting

Since about 99.9 per cent of the sequence is the same between people, DNA fingerprinting compares the variable parts: specific regions of repetitive DNA.

In repetitive DNA, a small stretch is repeated many times. In density gradient centrifugation it separates from bulk DNA: the bulk DNA gives a major peak, while the repetitive part gives small peaks, called satellite DNA.

Satellite DNA is classed as micro-satellites, mini-satellites and so on by base composition, segment length and number of repeats. It normally does not code for any protein, but it is highly polymorphic and forms the basis of DNA fingerprinting.

A DNA polymorphism is an inheritable variant (allele) at a locus that occurs in the population at a frequency above 0.01. Such variation is more likely in non-coding DNA, because mutations there have no immediate effect on reproductive ability and so build up over generations.

The technique was developed by Alec Jeffreys, who used a satellite DNA probe showing very high polymorphism, the Variable Number of Tandem Repeats (VNTR).

Steps: isolate DNA; digest it with restriction endonucleases; separate the fragments by electrophoresis; transfer (blot) them to a synthetic membrane such as nitrocellulose or nylon; hybridise with a labelled VNTR probe; and detect the hybridised bands by autoradiography.

VNTRs belong to the mini-satellite class: a short sequence repeated in tandem, with a copy number that varies from chromosome to chromosome and is highly polymorphic, so VNTR size ranges from 0.1 to 20 kb. The resulting band pattern is characteristic of each person and matches only between identical (monozygotic) twins.

Using the polymerase chain reaction (PCR) has raised the sensitivity so far that DNA from a single cell is enough for the analysis. Because the polymorphisms are inherited from parents, the method is used in paternity testing, forensic work and studies of population and genetic diversity.