The story behind DNA fingerprint

During early investigations into the variation and evolution of human genes, Sir Alec Jeffreys discovered a remarkable property concealed within human DNA. “We’d got to the point where we could detect single copies of human genes which led to one of the first observations of introns, non-coding sections of DNA that split up genes.

But when I came to Leicester in 1977, I wanted to move away from the study of split genes, and to marry the new techniques of molecular biology with human genetics,” he explains. His plan was to use gene detection techniques not only to look at the structures of genes but also to understand inherited variation between people.

“We knew about heritable variation in gene products such as blood groups, but we were looking for inherited variation at a far more fundamental level, namely in DNA itself.

An accidental breakthrough

The first examples of DNA variation proved to be rather uninformative and tedious to detect. The question therefore was whether far more variable regions of human DNA exist. The breakthrough came from a different project in Professor Jeffreys’ lab.  Professor Jeffreys’s plan was to use the primitive gene detection methods of the time to look at the structures of genes and understand inherited variation – the variation between people.

An early outcome of this research was one of the first descriptions of a restriction fragment length polymorphism RFLP. DNA-cutting enzymes target short DNA sequences, and chop the genome into pieces. Some people have a small DNA change – a single nucleotide polymorphism SNP – in a target site, preventing the enzymes cutting the DNA at that site.

While looking at the human myoglobin gene, which produces the oxygen carrying protein in muscle, he noticed an intron in that gene containing tandem repeat DNA short sequences repeated a number of times which became known as minisatellites. He reasoned that these minisatellites had the potential to be highly variable in terms of numbers of repeats.

“The true story of DNA fingerprinting starts at the headquarters of the British Antarctic Survey in Cambridge,” says Professor Jeffreys. “I collected a big lump of seal meat from their lock-up freezer and, to cut a long story short, we got the seal myoglobin gene, had a look at human myoglobin gene and there, inside an intron in that gene was tandem repeat DNA – a minisatellite.”

This minisatellite was to prove the key to the rest of the genome, for while it was not variable itself, its sequence was similar to the very few minisatellites that had been described previously.

Using the myoglobin minisatellite as a ‘hook’, the team could then identify more minisatellites and to their surprise discovered a core sequence – a piece of DNA that is similar in many different minisatellites.

“Using the core sequence, we made a probe that should latch onto lots of these minisatellites at the same time,” says Professor Jeffreys, “and, to test out the system, we hybridised the probe to a blot with DNA from several different people.”

On a Monday morning in September 1984, the X-ray of the blot was developed in the Leicester University darkroom. “I took one look, thought ‘what a complicated mess’, then suddenly realised we had patterns,” says Professor Jeffreys. “There was a level of individual specificity that was light years beyond anything that had been seen before.

The team identified more minisatellites and to their surprise discovered a core sequence – a piece of DNA that is similar in many different minisatellites. This chemical similarity allowed him to develop a method for detecting many minisatellites simultaneously.

He tested this idea on a panel of DNAs, and by accident produced the very first DNA fingerprint, a complex pattern of bands on an X-ray film. “I took one look, thought ‘what a complicated mess’, then suddenly saw we had some extraordinarily variable patterns,” he says. “There was a level of individual specificity that was light years beyond anything that had been seen before.

It was a ‘eureka!’ moment. We could immediately see the potential for forensic investigations and paternity, and my wife pointed out that very evening that it could be used to resolve immigration disputes by clarifying family relationships.”

That first X-ray film also had a range of animal and plant DNA samples, and showed that the same technique worked not just on humans, opening up exciting possibilities in conservation biology, ecology and wildlife forensics.

From the laboratory into the real world

It took only a few months for the technique to be refined into clean barcode-like patterns that allowed DNA fingerprints to be interpreted clearly. With highly automated and sophisticated equipment, the modern-day DNA fingerprinter can process hundreds of samples a day. Professor Jeffreys was able to develop practical testing methods using these DNA profiles to determine whether samples were from the same person, relatives, or non-related people.

Since his groundbreaking work in the mid-eighties, DNA analysis in forensic cases has become universally adopted and has meant a revolution not just in forensic investigation but in proving paternity and kinship, reuniting families as well as serving justice. DNA fingerprinting has thus become an indelible part of society, helping to prove innocence or guilt in criminal cases, resolving immigration arguments and clarifying paternity.

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