[MUSIC] Now, you remember Alec Jeffries. When he started doing this work, he required samples of material which were relatively large. But many traces are actually extremely small, and very small traces cannot be analysed using just those techniques. But there is now a method whereby DNA can be amplified, and this is called the polymerase chain reaction, or PCR. And this is how it works. Suppose you have length of double stranded DNA. When that DNA is heated to a temperature of 94 degrees centigrade, that is too hot for the hydrogen bonds. And all the hydrogen bonds break, and the two strands of DNA separate. Okay, now suppose you add primers, which are short lengths of DNA which match, they're complementary to the ends of these strands. And this of course, has to be done at a lower temperature. So the temperature's been brought down to 60 degrees, otherwise the hydrogen bonds wouldn't form. Okay, now at 72 degrees, an enzyme called DNA polymerase is added as well as the four nucleotides. The DNA polymerase will recognize the end of the primer, and it will build up the DNA chain, so that it is a complementary sequence to the original strand. So if we go through this sequence with the different temperature changes, at the end, we have double the amount of DNA that we started with. In fact, what we've done, is copy it. So if you do it once, you double the amount of DNA you have. But there's no reason why you can't do it more times. So you could go through the sequence once, twice, three times, four times, etc. Each cycle turns out to take about five minutes. And typically, it's done with 25 to 35 cycles, and it's done in a machine like this, which is called a thermal cycler. So what is the result of the PCR? It allows you to repeatedly duplicate the part of the DNA which you're interested in. And those 25 to 35 cycles of PCR will give you about 1 million copies. So if you have a very, very small trace from your crime scene, you can submit it to the polymerase chain reaction, and you can convert it a million times. This means very, very small amounts can provide DNA that can be analysed. So if you have just 36 cells, you can do this. And you can even go lower, maybe even 9 or so cells. But now, this is really a small number, and this is called low copy number, because 9 really is a very small number. But with PCR, you can now analyse really small traces. For instance, the saliva left on a cigarette end. Okay, remember the case back in our introductory lecture. You can analyze the saliva off the back of a stamp, or you could analyse the DNA contained in a blood stain that's barely visible.