DNA Analysis: are the crime shows accurate?
Forensic science involves the use of scientific methods in order to help within the court of law. DNA analysis is a critical tool within forensic science and helps solve real crimes and more importantly, crimes in TV shows. But, how accurate are these portrayals? How do we know we’ve got the right guy? So, let’s pretend we are forensic scientists and we have been called to the scene of a crime! DUN DUN.
Blood, hair and other DNA stuff
Okay so we get to the crime scene and we start looking for potential DNA samples. DNA or Deoxyribonucleic Acid is the stuff are the instructions that make up “you”. DNA is specific to each person (except identical twins). The ability to determine biological evidence at the crime scene allows crime investigators to have valuable information in order to convict. DNA is within every cell of your body (and there are trillions of cells). Skin cells, hair follicles, saliva, semen and blood are all common DNA samples found at the crime scene.
In order to analyse DNA, forensic scientists use short tandem repeats (STRs) within the DNA sequence. DNA has building blocks called “bases” and they are either A, G, C or T. In some places within the DNA genome, they repeat a lot like: TTATTATTATTA (TTA x4). This happens at various places within the genome and differ between individuals. So, at a particular spot in the genome, someone might have a TTA x4 repeat and another person has TTA x19 repeat. The combination of regions of STRs means every has a very unique DNA profile, with there being a 1 in a billion chance you have the same as someone else in the world. Australian scientists use 9 regions for this comparison. In the USA, they use 13 regions as the population is bigger and it there is more chance of similar STRs in regions. So, if you were a forensic scientist and the suspect’s DNA matched 9/9 with the DNA found at the crime scene. We can say that suspect was definitely at the crime scene and probably causing mischief.
At the crime scene
The first step of DNA testing is getting some DNA! Easy enough. Forensic scientists collect DNA samples from the crime scene as soon as possible buuuut only after putting on essential PPE. This includes are very fashionable looking full body protective suit which covers your hair and little booties to make sure you aren’t leaving footprints or other DNA through the crime scene. This step is typically forgotten in the crime shows, I guess the characters don’t look as cool in full PPE. It is also important for only the essential forensic scientists and crime scene investigators to be within the crime scene. Shows often have a hoard of people with clipboards running around the crime scene. In reality, that is super unorganised and messing with evidence. But it does make it dramatic… Whilst you are walking around collecting samples you might come across a dark red splotch. You would want to check if it’s blood, so we can use a Kastle-Meyer test or a Luminol test.
Kastle-Meyer involves a combination of chemicals; phenolphthalein and hydrogen peroxide. When swabbing some of the suspected blood with this test, the haemoglobin within the blood jump-starts a reaction with the two chemicals, turning the phenolphthalein pink and telling the crime scene investigators “this is blood”. Another test commonly shown on TV is the luminol test. Scientists spray luminol onto surfaces and the luminol reacts with hydrogen peroxide to create energy, the electrons become excited and emit a glow. One thing TV gets wrong with these tests is that they are usually used as a last resort due to the chemicals damaging the DNA. Luminol also doesn’t need a blue light to work, it glows on its own and all you need to do is make the room dark.
If there are several suspects within the trial, the forensic scientists also require suspect DNA samples to match against the DNA found at the crime scene. The scientists might also require elimination samples of people who help in the investigation or maybe known innocent people within the investigation. The elimination sample ensures the DNA analysis is only looking at the unknown sample and not a combination of other DNA. They collect these samples by a swab inside the cheeks.
In the lab
A major difference between real life forensics and the crime shows is the time taken to complete an analysis. Each step of the DNA analysis takes time and rarely are forensic scientists working on just one case! The lab can get busy and some samples have to wait. This is definitely not the case in crime dramas when the cool investigators burst into the lab and ask for results. Also, would an email be so terrible?
Extraction: So firstly, we need to get the DNA out of the cell so we can use it! At the lab, we will use a machine to open the nucleus of the cell and release the DNA as well as isolating the DNA, leaving behind any other material in the sample. The DNA is also cleaned to make sure it’s a pure sample of DNA used for further testing.
Time taken: minimum of 2-3 hours
Quantitation: So, this is a step to ensure we have enough DNA and if the DNA hasn’t been screwed up too much so that we can continue to analyse. The quality of the DNA is essential for correct matching later on. The amount of DNA is also extremely important as we need to make sure we have enough for the next step.
Time taken: 30-60 min set up and 2 hours to run
Amplification: This step essentially is to make more copies of the DNA we have. This is important as the crime scene sometimes has very limited DNA. This will then allow us to actually visualise the results and analyse the data! We amplify the DNA by using a process called PCR and we have a machine that heats and cools the DNA multiple time for 28 cycles to make sure we have enough DNA for the rest of the analysis.
Time taken: 30 minutes set up, 3 hours to run
Separation: The DNA sequences need to be separated so we can see actually differences between the DNA samples! This can be done through electrophoresis, which using an electric current to push the small molecules along the tray further than the bigger molecules. This provides visual results for the crime investigators to match against the suspects.
Time taken: 15 minutes for set up, 45 minutes to run
Total time to take to analyse DNA (with no interruptions, no lunch breaks, no other DNA needed to be analysed first, minus the paperwork involved and time at the crime scene and transportation of DNA, no analysis of the DNA and matching against the suspects) ≈ 10 hours
So, the crime shows aren’t perfect but they sure are dramatic! If you can look past the actors picking up crucial evidence with their own pens for some reason, they can be pretty fun!