The nuclei of all eukaryotic cells contain three RNA polymerases (designated I, II and III). These enzymes catalyze the formation of the phosphodiester bond that links nucleotides together to form a linear chain. RNA polymerase moves stepwise along the DNA, unwinding the DNA helix just ahead of the active site for polymerization, to expose a new region of the template strand for complementary base pairing. In this way, the growing RNA chain is extended one nucleotide at a time in the 5' to 3' direction. Each eukaryotic RNA polymerase catalyzes transcription of genes encoding different classes of RNA...