DNA fingerprinting opened new doors to an almost 100% accurate identification of individuals by comparing DNA that is unique to each individual. Forensics make use of this technology not only to identify potential suspects from evidence collected at crime scenes but also to identify crime and catastrophe victims. How does all this work then?
What is DNA?
|DNA molecule with its double-helix structure. Image source|
The DNA molecule (deoxyribonucleic acid) is basically composed of only four “building blocks” or nucleotides: Adenine, Thymine, Cytosine and Guanine. These molecules form a polymeric chain. Each of the nucleotides is bound via hydrogen bonds to its complementary nucleotide on a parallel chain (Adenine is always bound to Thymine, Guanine is always bound to a Cytosine) forming a double helix structure. The sequences that can be formed by just these four nucleotides is what defines each gene and all the inter-genomic regions. And it’s the differences in the inter-genomic regions that make each of us different and unique.
|Nucleotides bound to the complementaries. Image source|
It is now a long time since the first complete genome was sequenced. At only 5368 base pairs (a base pair is defined as one nucleotide bound to its complementary nucleotide), the sequencing on the Bacteriophage fX174 was a milestone in sequencing genetics and initiated a run that had its highlight with the publication of the 2.85 billion base pairs of the human genome.
|DNA fragments being extracted from an agarose gel. Photo by source.|
Until the moment that sequencing whole genomes can be done in a fast and cost-effective way (sequencing the human genome took more than a decade and millions of dollars to complete), DNA fingerprinting makes use of the variable regions between our genes that characterize our uniqueness.
Usually, a set of small probes that bind to specific DNA regions (markers) are used. By the patterns generated by these probes, an individual profile is created.
In criminal cases, the analysis involves gathering DNA from the crime scene and analyzing it for the presence of the markers thus creating a marker profile. This profile is then compared with profiles from suspects or compared against profile databases of convicted criminals. A match in only one of the markers is usually not uncommon but matching in five or more of these regions is enough to confidently say that both DNA samples came from the same individual.
And voila, there you have a very basic approach to what DNA fingerprinting is all about.
If I see some interest from you in wanting to know more about this, I’ll make a series of posts explaining the technology behind it and other ways in which genetics can be used as a tool in forensics.
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