A DNA molecule of length tethers a magnetic bead to a glass surface below a pair of…

PHYSICS · Multi-part · Fermi University Physics · Biophysics & Medical Physics · Macromolecules, membranes and cell architecture · Molecular machines, transport and bioenergetics

Forces, Machines and Signals, Ch Measuring one molecule: traps, tweezers and single-molecule fluorescence

Problem

A DNA molecule of length L=3.0 μmL = 3.0\,\mathrm{\mu m} tethers a magnetic bead to a glass surface below a pair of permanent magnets, at T=298 KT = 298\,\mathrm{K}. The bead's lateral position is tracked, giving an rms fluctuation ⟨δx2⟩1/2=150 nm\langle\delta x^2\rangle^{1/2} = 150\,\mathrm{nm}. ((a)) Treating the tether as an inverted pendulum of length LL under a nearly constant stretching force FF, find FF from the lateral fluctuation. ((b)) State why no external force standard is needed for this calibration.

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