So let's suppose you have a double stranded DNA molecule and in that double strand DNA molecule is a gene that you want to study. Now, before you can study that gene, you have to make many copies of that gene so that once you have many copies at your disposal, you can carry out a variety of different types of experiments with that particular gene of interest. Now, if you don't know what the sequence of nucleotides in that gene is, but if you do know where that gene is found, if you do know what the flanking sequence is, what the sequence surrounding that gene is, how exactly can you make many copies of just that gene found inside that DNA molecule? Well, in biochemistry there's a process known as the polymerase chain reaction or PCR that allows us to do just that. So the polymerase chain reaction is a very effective method by which we can actually amplify make many copies of a single DNA segment. And it's very useful because it allows us a very quick way to make millions or even billions of copies of a single DNA molecule.