transcription elongation

Once the copyist is past the title and into the body of the recipe, they settle into a rhythm: read a word, write a word, slide along. Transcription elongation is that steady middle phase — RNA polymerase, now committed, marching along the gene and adding one ribonucleotide after another to the lengthening RNA. It is the productive heart of transcription, where the bulk of the copy is actually made.

During elongation the polymerase holds a moving transcription bubble of about 13 unpaired base pairs. Inside it, roughly 8 or 9 nucleotides of the newest RNA stay base-paired to the template as an RNA-DNA hybrid; behind the enzyme the two DNA strands re-zip and the finished RNA peels away. With each step the enzyme reads the next template base, selects the matching ribonucleoside triphosphate, forms a phosphodiester bond at the RNA's 3' end, and advances one base. A bacterial polymerase manages tens of nucleotides per second. The enzyme stays attached over very long runs — a quality called processivity — but it can also pause, backtrack, and proofread: if it adds a wrong base it can reverse a step and clip off the error, giving transcription a modest fidelity (worse than replication, but good enough for disposable copies).

Elongation matters because it is not just blind copying. Pauses and backtracking are points where regulation, RNA folding, and (in eukaryotes) the coupled machinery that caps, splices, and tails the RNA all get their chance to act. In eukaryotes the tail of RNA polymerase II is dressed with factors that ride along during elongation, so the RNA is processed even as it is being written — copying and finishing happen hand in hand.