So let's suppose we have an RNA chain and we're extending that RNA chain. So we're building our RNA molecule. Now, so far in our RNA molecule we have N number of nucleotides. And let's suppose we want to add one more nucleotide. We want to add one more ribonucleotide triphosphate. Now, what RNA polymerase does and we'll discuss this in much more detail in just a moment what RNA polymerase does is it essentially attaches the ribonucleocide triphosphate onto the RNA molecule to form an RNA molecule that is extended by one nucleotide.