Transcription Bubbles

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During the elongation phase of RNA synthesis, transcription occurs within 'transcription bubbles' that form during the open complex stage. The elongation phase initiates once the first phosphodiester bond has formed. The loss of the sigma factor allows the core enzyme to bind strongly to the DNA template. The area containing RNA polymerase, DNA and the newly forming RNA forms the transcription bubble. This is a localised 'melting' of DNA that forms a bubble like structure in which DNA transcription occurs. The newly formed RNA forms a 'hybrid helix' with the DNA template strand. The transcription bubble moves rapidly along the DNA template and creates an elongation rate of '50 nucleotides per second'. The relative lengths of the DNA-RNA hybrid and the unwound DNA stays relatively constant throughout the movement of the transcription bubble indicates that DNA is unwound at the same rate as it is reformed [1].


  1. Berg J., Tymoczko J and Stryer L. (2007) Biochemistry, 6th edition, New York: WH Freeman.

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