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Forensic science is an older discipline than you may think. In fact, autopsies to determine cause of death have been around since ancient times. Since this time, the field has grown and advanced alongside developments in science.
One excellent example is DNA analysis, which was first used in 1986 during a criminal investigation. At that time crime scene investigators were only able to collect DNA from biological samples (such as bloodstains) that were larger than 2cm, roughly the size of a nickel. Samples this large were required because at that time large quantities of DNA were required to perform the laboratory tests required to identify someone from their DNA.
For these early laboratory tests, the sample from the crime scene was processed so that the cells in the sample would break open and the DNA inside the cells would come out. This DNA would then get mixed with a radioactive chemical (to help visualize it later) and then the pieces of DNA would be sorted by size using a process called gel electrophoresis. Gel electrophoresis takes advantage of the fact that DNA has a negative charge, so it can be attracted to a positive charge like a magnet. The DNA is added to one end of a gel, which has a texture similar to thick jello, and an electrical current is run through the gel so the DNA will start to move towards a positive charge. Bigger pieces of DNA have more trouble pushing through the gel, so they don’t move as far, and small DNA fragments travel further along the gel. This generates a distinctive appearance of stripes on the gel, which we call “bands” that scientists can see by exposing the gel to an X-ray. The bands represent the actual physical DNA, so different DNA sequences (like from different people who may have committed a crime) make different patterns of bands. This labor-intensive process required lots of DNA, and lots of time – 6-8 weeks in the early days!
Thanks to advances in the way we extract and amplify (make more of) DNA, forensic scientists can now extract DNA from biological samples smaller than a pinhead. Using a process called PCR (which stands for polymerase chain reaction) important sections of the DNA from inside a small number of cells can be multiplied to more than a billion copies. This means there is plenty of DNA to run tests on, even if you don’t start out with much in the sample! The DNA is then mixed with chemicals that light up under special lighting conditions and instead of running on a big gel can be run through a tiny tube using “capillary” electrophoresis. A detector at the end of the tube is able to detect the DNA lighting up, and from this determine how big the DNA fragments are – kind of like what a pattern of bands would look like, but much more automated than the system used in the 1980’s, and also much faster. A profile can now be generated in less than two hours!
The distinctive pattern of DNA found at a crime scene can now be easily compared to DNA profile databases around the world to help determine who was present at the crime scene. This advance in technology means that unsolved cases from the past may have another chance to be solved. Evidence from cold cases can now be examined for small DNA samples that would have been unusable in the past, amplified using modern technology, and then analyzed to create a DNA profile of whoever left the sample at the crime scene. There have been numerous high profile cold cases solved in this way in recent years. With further advances in this technology, it will be interesting to see what happens next for DNA analysis in the field of forensics.
To read more about the specifics of advances in electrophoresis you can click here.
You may also like to read one of our Quick Snips that focuses on advances in extracting DNA from hair.