Elongation and termination of transcription

The five subunits of RNA polymerase are shown as a single ellipse-like shape surrounding the transcription bubble. (a) Elongation: Synthesis of an RNA strand complementary to the single-strand region of the DNA template strand is in the 5′-to-3′ direction. DNA that is unwound ahead of RNA polymerase is rewound after it has been transcribed. (b) Termination: The intrinsic mechanism shown here is one of two ways used to end RNA synthesis and release the completed RNA transcript and RNA polymerase from the DNA. In this case, the formation of a hairpin loop sets off their release. For both the intrinsic and the rho-mediated mechanism, termination first requires the synthesis of certain RNA sequences.