Fig. 5. A transcription cycle. A chromatin loop is attached through transcription factors to a factory; these attachments are made and broken continuously as components in the factory exchange with the soluble pool. On initiation, the promoter (white circle) binds to a polymerase in the factory, and the transcript is extruded as the template slides (white arrows) through the polymerase; on termination, the template detaches. Between initiation and termination the template is bound to the factory both stably (the bond is one of the stablest noncovalent ones known) and persistently (it takes 5 minutes to transcribe a typical human gene) (Kimura et al., 2002). Therefore, such attachments are much more stable and persistent than those mediated by transcription factors, and this may underlie the difference in `strength' between transcribed and nontranscribed enhancers.

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