The RNA polymerase holoenzyme binds to a promoter of an exposed DNA strand and begins to synthesize the new strand of RNA. The double helix DNA is unwound and a short nucleotide sequence is accessible on each strand. The transcription bubble is a region of unpaired bases on one of the exposed DNA strands. The starting transcription point is determined by the place where the holoenzyme binds to a promoter. The DNA is unwound and single-stranded at the start site. The DNA promoter interaction is interrupted as the RNA polymerase moves down the template DNA strand and the sigma factor is released. The σ factor is required for the initiation but not for the remaining steps of the DNA transcription. Once the σ factor dissociates from the RNA polymerase, the transcription continues. About 10 synthesized nucleotides of a new RNA strand are required for this to proceed to the elongation step. The process of transcribing during elongation is very fast. Elongation takes place until the RNA polymerase comes across a termination signal (terminator) which arrests the process and causes the release of both the DNA template and the new RNA molecule. The DNA usually encodes the termination signal.