When the RNA transcript reaches 7 nucleotides long, transcription enters the elongation phase, the beginning of which is characterised by the collapsing of the DNA bubble and the ejection of TFIIB. This is thought to be because the nascent RNA clashes with the B linker helix when it is 6 bases long and upon further elongation to 12-13 bases it will clash with the B-reader and B-ribbon leading to dissociation. The DNA duplex also clashes with the B linker above the rudder (caused by rewinding of the DNA into a double helix).