Figure 3

The use of LNA toehold probes in magnetic tweezer experiments. (a) Force-extension curves of 4.8 kb DNA tethered with an LNA toehold to the glass surface, showing the overstretching plateau at around 65 pN. Of these beads 44% ruptured before overstretching, 22% during the 4 seconds of overstretching, and 34% of the tethers stayed intact after overstretching (N = 176). The black line represents a worm-like chain (WLC) model for DNA of 4.8 kb, and 3 individual molecules are shown in color. (b) Rupture of beads at 35 pN was fit with a double exponential decay. A total of 238 beads were monitored, and 76 ruptures were detected within 2 minutes. (c) Schematic representation of an 18S chromatin molecule immobilized on the glass surface by the LNA toehold probe and 3 LexA-TAP proteins to an IgG coated bead. (d) Force extension curves of native chromatin. Each curve represents a single fragment isolated from the yeast extract. (e) Discrete steps in extension of 25 nm (dotted lines), characteristic for the unwrapping the DNA of the inner turn from a single nucleosome. (f) Histogram of the steps of the inner turn unwrapping and fit, yielding a step size of 24.8 ± 0.1 nm (mean ± S.E.M.).