Now, we know that DNA molecules are very long molecules. They consist of many, many nucleotides. The question is how exactly does the RNA polymerase know where to begin the process of transcription on that DNA molecule? So if we have many of these genes and many of these nucleotides how exactly does the RNA polymerase know where to bind and begin the process of transcription of some particular gene of interest? Well, basically in prokaryotic as well as eukaryotic cells we have these regions on the DNA known as promoter regions or promoter sites. And what these promoter sites are, there are specific sequences of nucleotides that can bind very well to the RNA polymerase.