Figure 2.

Intensity noise affects trapped bead position (x_bd) and measurements of DNA contour length (L). (A) Unwanted bead motion (δx_bd) occurs under constant load (F) when an intensity fluctuation (δI) decreases the trapping potential. (B) A one-dimensional diagram of the experiment and its mechanical analog. Variables represent the following: k_DNA is the stiffness of the DNA molecule, k_T is the trap stiffness, x_stage is the distance between the tether point and the trap center, x_DNA is the end-to-end extension of the DNA, and r_bd is the bead radius. (C–E) Based on a first-order calculation (Eq. 8), the uncertainty in DNA contour length (δL) is linear in intensity fluctuation (C) and DNA length (D) but independent of k_T at constant force (E). Calculations were performed with a constant trap force of 6 pN and r_bd of 165 nm. When parameters were not varied, they were held fixed at the following values: dI/I = 1%, k_T = 0.086 pN/nm, k_DNA = 0.187 pN/nm, x_bd = 70 nm, and L = 1000 nm.