Once transcription is initiated, the DNA double helix unwinds and RNA polymerase reads the template strand, adding nucleotides to the 3′ end of the growing chain (Figure 2b). At a temperature of 37 degrees Celsius, new nucleotides are added at an estimated rate of about 42-54 nucleotides per second in bacteria (Dennis & Bremer, 1974), while eukaryotes proceed at a much slower pace of approximately 22-25 nucleotides per second (Izban&Luse, 1992).