2023-04-21 02:02:33
Criminals beware! The smallest residue of saliva on the Coke bottle and cigarette filter, a few skin cells under the fingernails or bloodstains on the victim’s clothing – all of this can be enough to convict a perpetrator using his genetic fingerprint.
DNA leftovers everywhere
In all of these cases, the telltale starting point is traces of genetic material left behind at the scene of the crime. Because most of our bodily fluids also contain cells of our body – and with them also our genetic information. And when we rub our hands along a rough surface or are scratched, skin cells always get caught – and with them our DNA. If we lose a hair, a bit of genetic material remains on the ground with the hair root.
These genetic remains are a great help for investigators. You can tell them if a suspected person was really the perpetrator. However, there is a catch: there is very little genetic material in the traces of the crime scene – actually too little to decode the crucial DNA sections using classic methods. For a long time, this was the reason why DNA analyzes from such relics were impossible.
Only the polymerase chain reaction makes it possible
However, that changed with the American chemist Kary B. Mullis. In 1983 he came up with an idea of how the few traces of DNA could be multiplied – and invented one of the most important methods in genetics and biotechnology to date: the polymerase chain reaction (PCR). It still forms the basis for DNA tests today, but also for the detection of a virus infection using viral DNA and RNA – during the corona pandemic, “PCR test” became a common vocabulary.
The principle of the PCR is very simple: A DNA fragment up to 3,000 base pairs long is first heated to 94 to 96 degrees Celsius. This breaks the hydrogen bonds that hold the bases of the double strand together. As a result, the double helix separates into two single strands. Two so-called primers are then added to the DNA solution. They accumulate at certain points in the DNA sections and serve as a starting signal for the next step – copying.
This is where a heat-stable enzyme, the polymerase, comes into play. At a temperature of 60 to 70 degrees, it links DNA building blocks floating around freely in the solution in such a way that they form an exact copy of the sequence marked by the primer. By repeating these steps umpteen times, the number of DNA fragments multiplies exponentially. Once the PCR has gone through, the few relics from the crime scene have turned into a solution with millions of copies of the DNA.
Individual repeat pattern
Now the actual DNA test can begin. Scientists do not compare the entire genome sequence for this – that would be far too complex and time-consuming. Instead, they only look at short excerpts of it. They are located in the non-coding sections of the genome and consist of repeated sequences of bases, the so-called “Short Tandem Repeats” (STR). The STRs vary in number from person to person and are therefore an individual genetic fingerprint.
For the DNA test in the crime laboratory, around a dozen STR systems on different chromosomes are usually examined, as well as a sex-distinguishing feature. This is usually enough to rule out a coincidental match in the DNA profile. Because our individual STR pattern only matches one in a billion people. If the genetic fingerprint of the crime scene and the suspect match, it is very likely that it is the perpetrator.
Paternity test: is it a “cuckoo child”?
The paternity test works in a similar way. Because the family relationship can also be determined via DNA comparisons. The procedure is very simple: A saliva sample, a few hairs with hair roots, the baby pacifier covered with spit or a piece of chewing gum that has been well chewed is enough. The DNA is then first isolated from these samples in the laboratory and multiplied by means of PCR. Then it’s on to the actual comparison test. Genetic samples from the child and the father, and preferably also from the mother, are required for this.
The DNA paternity test does not compare the entire genome either, but only the sequence of bases of around 15 to 20 sections. Like the forensic DNA test, these are short tandem repeats. How often a specific base sequence is repeated within such an STR marker is inherited from parents to children. At each locus they carry two variants of such markers – one from the mother and one from the father.
The odds of two unrelated people having the exact same pattern of repeats at these markers is about one in 100 billion, according to current estimates. Conversely, paternity or maternity is ruled out if the genome of parent and child differs at three or more STR markers.
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