Prokaryotic DNA transcription: elongation
The elongation phase of transcription refers to the process through which nucleotides are added to the growing RNA chain. As the RNA polymerase moves down the DNA template strand, the open complex bubble moves also. The bubble is of a fixed number of nucleotides, meaning that at the leading end of the bubble the DNA helix is being unwound, while at its trailing end the single strands are being rejoined. Whereas separation of the DNA helix is permanent in replication, it is only temporary in transcription. depicts the beginning steps in transcription up to elongation and the relative positions of the bubble and the polymerase holoenzyme.Figure %: Steps in TranscriptionAs the figure shows, within the open complex bubble the DNA and RNA form a hybrid or joint complex. The exact length of this region is unknown, but it is thought to be between 3 and 12 base pairs long and is found at the growing 3' end of the RNA. The figure also illustrates how the 5' tail end of the RNA chain is separate from, as opposed to base paired to, the DNA template strand. This is another difference between DNA replication and DNA transcription; in replication, the newly synthesized DNA strand remains bound in a helix to the strand with which it has base paired. After the initial stretch of approximately 8 base pairs has been synthesized, the sigma unit, which is responsible for recognition and binding to the promoter region, is released. The core enzyme is left to polymerize the growing RNA chain alone. This leads to the continuous extrusion of the 5' end of the RNA from the enzyme complex. At normal room temperature, the rate of transcription in prokaryotes is 40 nucleotides per second.