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Behind the Crime Scene: How Biological Traces Can Help to Convict Offenders


Although cutting-edge forensic laboratories feature frequently in films and television, how many of us know what actually goes on there in real life? Let’s take a look at the work of forensic scientists and find out how hair, saliva or blood can help to solve criminal cases.

DNA – invisible but unique

DNA analysis has revolutionized forensic medicine since the 1980s with DNA being found on every crime scene. People, inadvertently, leave their DNA wherever they go, embedded in skin scales or hair for example. It is the role of forensic specialists to identify genetic traces and to attribute these which might belong to suspects, victims or to other innocent third parties.

DNA is the material in which our genetic information is stored. The base sequence of DNA is as unique to each person as their appearance or personality. Every cell of our body carries this genetic material. However, only around 5% of this DNA comprises “true genes” which provide the code for proteins. Most DNA belongs to so called non-coding DNA, a major part of this being in the form of repetitive sequences.

In a TV thriller, forensic experts often come to the scene of a murder or other major crime, collect evidence and take it to their laboratory. A few hours later a detective calls them to discuss the DNA results. This is pure fiction - in reality, it’s a long and extensive process to examine DNA traces.

How to prepare a DNA profile

Firstly, it’s necessary to identify the DNA material. Although, this can be a simple process in the case of blood or chewing gum deposits, it can often be less straightforward. Sometimes traces are so small that they are not even visible to the naked eye. In such instances, forensic experts must use cotton swabs or adhesive tape to collect evidence and identify any biological material. Through a follow up process, DNA is then extracted using specific chemicals. Finally, certain DNA parts from non-coding regions are amplified by an enzymatic procedure called polymerase chain reaction, or PCR.

When forensic scientists prepare a DNA profile it is not about analyzing the genetic information. The DNA fragments gained from the biological traces will only provide a genetic fingerprint which is of no direct value in itself. The only practical way to identify the person to whom the DNA belongs, is to match the results with existing DNA records on official databases. Correct identification can only occur when a DNA sample has previously been collected from the persons concerned.

A more in-depth view

Forensic experts can learn more from biological traces. Under the microscope, they can examine sex chromosomes and determine whether samples are male or female in origin. When body fluids are available they can also analyze the hormone status of individuals and confirm, for example, whether a woman has been pregnant and, if so, for how long.

Toxicological analysis is also undertaken by forensic laboratories. From investigation of hair or saliva samples, they can determine whether poison or drugs might have played a role within a crime.

Many options for the future

Even in the future, DNA testing alone will not lead to the conviction of offenders. Scientists, however, are currently trying to find out about the relationship between DNA sequences and physical appearance. They have already identified specific DNA locations which are responsible for determining hair, eye and skin color. Nevertheless, one thing is clear: it will still be a long time before forensic experts can draw an identikit picture from DNA information on a cigarette butt found at a crime scene.