How does DNA profiling work?

DNA is seen as the gold standard in forensic science, but is it really all it's cracked up to be?
03 May 2016

Interview with 

Dr Mari Uchimoto, Anglia Ruskin University


DNA evidence is seen as the gold standard in forensic science, often being used to clear DNAor condemn suspects, but what makes this type of evidence so important, and is it really infallible? Georgia Mills went to visit Dr Mari Uchimoto in her DNA lab at Anglia Ruskin University... 

Mari - The DNA that you get a crime scene is probably not going to be the cleanest sample. It might be off the floor, it might be off the toilet floor so, one of the first steps what we have to do is extract and purify it.  Also, to get rid of any contaminants that might be present, as well as cellular material. The second step would be to find out how much there is, so it's called quantification. The amount of sample that your normally have at a crime scene is usually very limited in both quality and quantity, so it's necessary to amplify that so. So, basically, get more of that and many, many copies.

Georgia - Mari amplifies the DNA with what is called PCR. DNA is double stranded - so imagine a train track. And when you heat the DNA, the train tracks split apart, and then, using some proteins called primers, this can rebuild each side of the split track, meaning you now have two train tracks! This is repeated over and over until you have billions of pieces of this original strand of DNA!

Mari - Once we've done that we can actually separate it and find out what fragments we actually have. So we use a  what's called a capillary electrophoresis machine and that can generate a DNA profile, and hopefully that's a full DNA profile.

Georgia - This machine in the lab looked a bit like a computer from the 90s, big grey and blockish, the front opened up to reveal several tiny glass tubes, which all  led into a small black box with a sign saying "Danger - avoid direct exposure to beam". So, the next stage is to take your DNA strands, which have been dyed a colour, and feed them into the machine...

Okay. So we're turning on the  - what's it called again?

Mari - It's called a - this model is a 3130. It's a genetic analyser and it's also used to separate fragments for DNA to produce a DNA profile, but it's also used for things like sequencing as well. So, it has a couple of different applications.

Georgia - It draws up the samples through all these tubes in a little labyrinth, puts it through this laser, which we've been warned very helpfully not to expose ourselves to, and then this laser if it detects the colours that have already been dyed, it will know here is the presence, or absence of a specific part of the gene?

Mari - Yes, yes, correct. And then, eventually, that will just pass through into the waste and then the information from the laser will then be transferred onto a data collection software where we can actually view the profile.

Georgia - Mari showed me one of the  DNA profiles they made - this isn't your entire genetic sequence - it's a set of, in this case, 17 sections of your barcode which are likely to be different in unrelated individuals. To my untrained eye it looked like a graph with small coloured spikes and these profiles from the crime-scene DNA can be compared to the profiles of the suspect's DNA - either incriminating or exonerating them. And the chances of a match with a stranger? About 1 in a billion. So, unless you have a particularly criminal identical twin, not bad odds.

Mari - Very, very accurate. Chances of having someone not being a match - 1 in a billion, so I think that's highly discriminatory. It's also very sensitive - you only need about 0.01 nanograms per microlitre. We can get a lot of data from that.

Georgia - Is there not a risk when every human has their own set of DNA that it could get quite easily contaminated?

Mari - Yes. Because the technique - DNA profiling- has become so sensitive that is always a constant worry. So, in the lab, we often wear lab coats; we completely clean down the surfaces; we generally have what are called pre-PCR and post-PCR labs,  just to make sure that we minimise the contamination, and obviously wear gloves, all the protective gear, tie your hair back, everything that you can think of.

Georgia - Dr Mari Uchimoto there.

Mari assured me that any accidental contaminations can be sorted out relatively easily - our prisons aren't full of genetic analysts. But, if  DNA can be so easily transferred, can the same be said for other types of forensic evidence, especially as technology advances means we're getting so much better at detection?


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