elongation and termination
/ ee-long-GAY-shun and ter-min-AY-shun /
Transcription is often described in three acts: getting started, the long middle, and stopping. Initiation is the start (the polymerase finding the promoter and opening up). This entry covers the other two acts. Elongation is the long stretch where the polymerase actually builds the RNA, and termination is how it knows to quit and let go at the end.
During elongation, RNA polymerase travels steadily along the template strand, adding RNA letters to the growing chain at a rate of roughly tens of letters per second. It keeps the transcription bubble open in front of itself, threads the new RNA out through a channel in the enzyme, and re-closes the DNA behind. The process is processive — the polymerase clamps on tightly and can copy thousands of letters without falling off — and it is reasonably accurate, with some ability to back up and correct a wrong letter, though less rigorously than DNA copying.
Termination is the signal to stop. Bacteria use two main tricks: in one, the RNA folds back on itself into a hairpin that physically pops the polymerase off; in another, a protein called Rho catches up to the polymerase and dislodges it. Eukaryotes mostly cut the RNA at a defined signal and then let the polymerase run off afterward. However it happens, the end result is the same: the polymerase releases the finished RNA and the DNA, and is freed to transcribe another gene.
Think of writing a long sentence: the steady, letter-by-letter writing is elongation; reaching the period and lifting your pen is termination. In a bacterium, that 'period' can be an RNA hairpin that flips the polymerase off the DNA the moment it forms.
Elongation is the steady writing; termination is the period that releases the polymerase.
Transcription proofreading is real but weaker than DNA replication's — RNA errors are tolerated because each RNA is a short-lived, disposable copy, not the permanent archive.