It also allows us to make copies of a gene or DNA segment that is relatively long, about 10,000 nucleotides in length. And it also allows us to make the copies without actually knowing what the sequence of nucleotides in that gene is, as long as we know what the flanking sequence is. So we have to know what the flanking sequence is because we have to build DNA primers for the DNA replication process, as we'll see in just a moment. So what do we mean by a flanking sequence? So let's take a look at the following double transit DNA molecule. Let's suppose you're given this molecule and this is the gene that we actually want to amplify.