Practice question

Transcription induces twin supercoiled domains because elongating polymerase cannot freely rotate around DNA helix. Positive overwound supercoils accumulate ahead of polymerase creating torsional stress opposing unwinding, while negative underwound supercoils trail behind. DNA gyrase, a type IIA topoisomerase unique to bacteria introducing negative supercoils using ATP-driven double-strand passage, preferentially removes positive supercoils in front of transcription complex. Topoisomerase IV also contributes chromosome decatenation. Topo I handles negative behind. This division of labor ensures polymerase advancement unimpeded by excessive supercoiling accumulation that would otherwise stall elongation or promote R-loop formation.