The Experiment: What's in a sausage?

10 October 2017

Interview with

Ed Farnell, University of Cambridge

So we can get DNA out of a strawberry in under a minute. But why stop at a strawberry? We wanted to know if we could get DNA out of a sausage, and whether we could read the sequence of genetic letters - or bases - in that DNA to work out what meat was in there. We put geneticist Stevie Bain on the case...

Stevie - I’m looking to buy some sausages. I’ll go for a couple of the pork ones please.

So the butcher claims that these sausages are 100% pork, but can we test this using DNA. I head to the Pathology Lab at the University of Cambridge where Dr Ed Farnell will help me figure out exactly what’s in these sausages. The first thing we have to do is extract the DNA from the sausage and to do this we use essentially the same techniques described in the strawberry experiment… albeit with slightly more expensive chemicals and fancier equipment…

Ed - We’ve got our DNA extracted now and we’re going to start preparing that DNA for sequencing. But instead of sequencing the entire genome of the pig and whatever else might be in the sausages, we’re going to focus on a small region which acts as a barcode for each animal. Now the way we’re going to target that region is using primers and these are short stretches of DNA which target our enzymes, and then the enzymes are going to make lots and lots of copies of it. Then that will allow us to make a library which we can put on the sequencer and get the sequence of those regions.

Stevie - To generate many copies of DNA fragments in this way, we require a technique called the Polymerase Chain Reaction or PCR which uses an enzyme called DNA Polymerase…

Ed - To amplify DNA by Polymerase Chain Reaction or PCR, you need to slip the two DNA strands. You do that be heating them up to 98 degrees celsius and that splits them in two. And then we cool it down to what we call our annealing temperature, which here is going to be 63 degrees celsius; that allows the primer to specifically stick to the bit of the chain. Then once it’s stuck we heat it back up to 72 degrees; that then allows the polymerase to come in and make a copy of the little region that we’re interested in.

Stevie - The PCR process takes around one hour to generate millions of copies of our DNA fragment. Once that’s done, a liquid containing special tags is then added to the DNA. These tags are important because they mark each of the DNA bases A, T, C, and G with a different colour which allows them to be read by a machine called a DNA Sequencer.

Ed - I’m stood in front of the machine right now, and it’s got a screen on the front and that’s where we do our interaction from. It’s touch screen so it’s really, really simple so I’m going to hit the nice big friendly button that says “sequence”. At that point, it’s going to grab our library that we’ve made which is basically our little copied bit of copied DNA with these tags on. Then you have a computer and a piece of software at the back that takes a picture every cycle and then reassembles that into a string of DNA letters. Off we go…

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