<P> Once the initiation complex is open, the first ribonucleotide is brought into the active site to initiate the polymerization reaction in the absence of a primer . This generates a nascent RNA chain that forms a hetero - duplex with the template DNA strand . However, before entering the elongation phase, polymerase may terminate prematurely and release a short, truncated transcript . This process is called abortive initiation . Many cycles of abortive initiation may occur before the transcript grows to sufficient length to promote polymerase escape from the promoter . Throughout abortive initiation cycles, RNA polymerase remains bound to the promoter and pulls downstream DNA into its catalytic cleft in a scrunching - kind of motion . </P> <P> When a transcript attains the threshold length of ten nucleotides, it enters the RNA exit channel . The polymerase breaks its interactions with the promoter elements and any regulatory proteins associated with the initiation complex that it no longer needs . Promoter escape in eukaryotes requires ATP hydrolysis and, in the case of Pol II - phosphorylation of the CTD . Meanwhile, the transcription bubble collapses down to 12 - 14 nucleotides, providing kinetic energy required for the escape . </P> <P> After escaping the promoter and shedding most of the transcription factors for initiation, the polymerase acquires new factors for the next phase of transcription: elongation . Transcription elongation is a processive process . Double stranded DNA that enters from the front of the enzyme is unzipped to avail the template strand for RNA synthesis . For every DNA base pair separated by the advancing polymerase, one hybrid RNA: DNA base pair is immediately formed . DNA strands and nascent RNA chain exit from separate channels; the two DNA strands reunite at the trailing end of the transcription bubble while the single strand RNA emerges alone . </P> <P> Among the proteins recruited to polymerase are elongation factors, thus called because they stimulate transcription elongation . There are different classes of elongation factors . Some factors can increase the overall rate of transcribing, some can help the polymerase through transient pausing sites, and some can assist the polymerase to transcribe through chromatin . One of the elongation factors, P - TEFb, is particularly important . P - TEFb phosphorylates the second residue (Ser - 2) of the CTD repeats (YSPTSPS) of the bound Pol II . P - TEFb also phosphorylates and activates SPT5 and TAT - SF1 . SPT5 is a universal transcription factor that helps recruit 5' - capping enzyme to Pol II with a CTD phosphorylated at Ser - 5 . TAF - SF1 recruits components of the RNA splicing machinery to the Ser - 2 phosphorylated CTD . P - TEFb also helps suppress transient pausing of polymerase when it encounters certain sequences immediately following initiation . </P>

What phase of transcription occurs when double stranded dna is separated