Figure 2: Force calibration and force-extension measurements in the FOMT.

(a) (x,y) Fluctuations (black) of a bead tethered by a 20.6-kb DNA construct under the cylindrical magnet with the magnet at a distance of 2 mm from the flow cell surface, corresponding to a high stretching force (8.5 pN). The fluctuations lie on a circular annulus (red line). (b) (x,y) Fluctuations (black) and fitted circle (red) for the same tethered bead as in (a) with the magnet at a distance of 6 mm from the flow cell surface, corresponding to a lower stretching force (1.5 pN). (c) Histogram of the fluctuations in the radial coordinate for the data from (a). The red line is a Gaussian fit with s.d. of σ=0.0542 μm. (d) Histogram of the fluctuations in the radial coordinate for the data from (b). The red line is a Gaussian fit with s.d. of σ=0.133 μm. (e) Force-extension data for DNA tethers of different length stretched using the cylindrical magnet geometry. Forces and DNA extensions were calibrated as described in the main text. Data are for 3.4 kb DNA using 0.5 μm diameter MagSense beads, and for 7.9 and 20.6 kb DNA using M270 beads. Extensions were scaled by the contour length to facilitate the comparison of the different length molecules. The lines are fits of the WLC model¹⁰. The inset shows the same data with a logarithmically scaled force axis. (f) Stretching forces for M270 beads as a function of distance of the cylindrical magnet from the flow cell surface. The flow cell used in these experiments has a thickness of ≈400 μm. Experimental data for 7.9 kb tethers and 20.6 kb tethers. The black line is a prediction from first principle calculations taking into account the magnetic field and the beads' magnetization (Supplementary Figs S11 and S12). The inset shows the bead and tether geometry used for force and length determination.