Dr Andy Hopwood discusses creating DNA profiles

Andy:
When we get samples submitted to the lab, first of all it's very, very important that we always write down where the sample is, what's happening to the sample. It's what we call continuity. So when we go to court we can always tell the judge, and the jury, and the barristers, exactly what's happened to the sample. And that's right from it coming into the lab to it going out of the lab at the end with a statement of evidence.

For DNA testing what we do is dissolve the crime stain in a liquid and we then purify the DNA from the liquid. We can then quantify the DNA so we can measure how much DNA is in that sample and then we use a process called "pulimarised chain reaction" and this is a very very important piece of technology. This has allowed us to make the DNA tests far far more sensitive than they have been in the past. And what it does is identifies the bit of DNA we want to grow and we can make billions and billions and billions of copies of a single piece of DNA – the target DNA that we're looking for. It kind of works like a bit of a molecular photocopier if you like.

The bits of DNA that we are looking at vary in length between individuals and it's that length variation that we're looking for. So what we do is once we've got these products amplified, we run them down a gel that separates them on the basis of size so you see on the gel effectively a ladder of different DNA molecules and we measure the size of those and we assign them a number based on their size and then that's what we call the DNA profile.

Lucy:
And so you get the sample from the criminal and get the sample from the crime scene and you compare the length.

Andy:
Yeah, we do. The length is designated a particular number and for each of the sites that we're looking at you expect to get two bands, one band from each of your chromosomes, and they might be the same size so you might see just one peak or they might be different – depending on what you've inherited from your parents.

Lucy:
Okay

Andy:
Okay. In the UK and in New Zealand we look at ten of those sites at the same time and the chances of your DNA matching an unrelated person is somewhere in the region of one in a thousand million.