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DNA melting is local or extensive separation of the two strands when stabilizing base pairing and stacking no longer maintain a duplex. Temperature, chemical environment or proteins can favour opening; a transcription initiation bubble is a localized example.

In bacterial transcription initiation, the RNA polymerase holoenzyme uses a promoter-specific sigma factor to recognize promoter elements and form a closed DNA-bound complex. Subsequent local DNA melting opens a transcription bubble, positioning the template strand in the active site: this is an open promoter complex. The enzyme can then initiate RNA synthesis and escape into elongation. LacI, the lac repressor, binds the lac operator overlapping or close to the promoter and can prevent productive promoter binding or its progression to an open complex; operator arrangement and DNA looping can strengthen repression. One should not assume every repressor acts at exactly the same microscopic step.

A high-resolution optical tweezers assay mechanically connects one trapped bead to RNA polymerase and another bead to an upstream or downstream DNA handle. Polymerase motion changes the tether length; measuring bead displacement and the calibrated restoring force reports elongation under a known load. A feedback force clamp can keep that load approximately constant. Reversing the tether geometry permits assisting and opposing loads. The left panel below shows the mechanical arrangement rather than a copied apparatus image.

A base-pair advance is only about $0.34\mathrm{nm}$ along B-form DNA. Resolving that increment requires exceptional mechanical stability, suppression of drift and laser-pointing noise, calibration of trap and tether compliance, and sufficient temporal sampling to separate steps from thermal bead fluctuations. Position histograms, dwell detection and their spatial correlations can expose a repeated increment; one should also test nucleotide-dependent motion, pauses and backtracking. Stabilized dual-trap measurements combined with transcription-product analysis established approximately one base-pair advance per incorporated nucleotide.^[ref]

Magnetic tweezers apply force and controlled turns to a single torsionally constrained DNA tether joining a surface and a magnetic bead. A magnetic-field gradient pulls the bead; rotation of the field rotates the bead and changes imposed turns. Imaging measures bead height, hence DNA extension. At sufficient excess twisting and suitable force, DNA supercoiling creates writhed loops that shorten the tether. The extension-versus-turn curve supplies a calibration relating the removal or addition of supercoiling to vertical motion. Multiple attachments at both ends are needed conceptually to constrain rotation; freely swivelling DNA would relax the signal.

During transcription initiation, RNA polymerase binds a promoter in a closed complex and then opens a local DNA bubble, exposing template for RNA synthesis. In bacterial initiation a sigma factor helps recognize the promoter. Opening changes local helical twist. Because the total linking number is constrained,Unwinding the bubble means $\Delta T{w}_{\mathrm{bubble}}<0$, so the remainder acquires a positive turn increment. A negatively supercoiled tether therefore loses some negative writhe and becomes longer; a positively supercoiled tether gains positive writhe and becomes shorter. Ordinary DNA compaction instead shortens both tethers. Comparing both signs separates local unwinding from compaction rather than assigning every height change to opening.

If $q$ is the number of turns unwound, $s$ the magnitude of the extension change per imposed turn, and $c$ the compaction, the symmetric-slope approximation predicts $\Delta z_{-}=sq-c$ and $\Delta z_{+}=-sq-c$. HenceMeasured unequal slopes can be retained in the two separate calibration equations. Multiplying $q$ by the helical repeat converts turns to an estimated number of unwound base pairs, subject to the mechanical model and the distinction between twist and writhe.

Repeated extension transitions reveal open-complex lifetimes, closed-complex lifetimes and their dependence on mechanical torque. Initial short RNA synthesis can occur while RNA polymerase remains associated with the promoter, including abortive initiation and DNA scrunching. Addition of nucleotides allows progression to promoter escape; an unwound bubble can then persist with elongation, so loss of the bubble is not the definition of escape. Successive cumulative unwinding events can identify renewed access to the same promoter after clearance. These are the measurement principles demonstrated by the paired-sign experiments.^[ref]

The experiment observes mechanics rather than directly imaging individual opened base pairs. Drift, nonspecific binding, tether integrity, force-dependent buckling and limited time resolution can mimic or obscure steps. A control lacking productive promoter recognition and agreement of the opposite-sign responses help identify the origin of a signal.