Now, what our RNA polymerase does is it binds onto that DNA molecule and it reads the DNA template from the three to the five end and that means it builds it from the five to three end. So this is the RNA chain that the RNA polymerase already built. So let's imagine that inside this chain we have N number of nucleotides and this is the N plus one nucleotide that is being added onto that growing RNA chain. So what the RNA polymerase does is it essentially hovers around this section and it brings that complementary ribonucleocide triphosphate which is complementary to this base that is found on the DNA template. Now, how do we know when it's complementary? Well, basically when the bonding is just perfect, this molecule will stay in this location for long enough for this bond to actually take place.