Torsional stress in linear biopolymers such as DNA and chromatin has important consequences for n... more Torsional stress in linear biopolymers such as DNA and chromatin has important consequences for nanoscale biological processes. We have developed a new method to directly measure torque on single molecules. Using a cylindrical magnet, we manipulate a novel probe consisting of a nanorod with a 0.1 μm ferromagnetic segment coupled to a magnetic bead. We achieve controlled introduction of turns into the molecule and precise measurement of torque and molecule extension as a function of the number of turns at low pulling force. We show torque measurement of single DNA molecules and demonstrate for the first time measurements of single chromatin fibers.
Movie S1-PROBE ROTATION. Real time movie of the rotation of the nanorod-bead probe by translation... more Movie S1-PROBE ROTATION. Real time movie of the rotation of the nanorod-bead probe by translation of the stage. The probe was tethered to the glass surface of the capillary tube by a 3 µm DNA molecule and lifted using the cylindrical magnet. Movement of the stage in a path around the projection of the center of the cylindrical magnet allows the nanorod-bead probe to be rotated in a controlled manner. Movie S2-NANOROD-BEAD PROBE. Fluctuations of the nanorod-bead probe under a vertical magnetic field created by a cylindrical magnet. Nanorod-bead probe was tethered to the glass surface by a 3µm DNA molecule. The vertical magnetic field lifts the probe while providing a weak horizontal angular trap. The horizontal angular fluctuations in this case spanned ≈100°. The left side is a bright field movie. The right side is the same movie as a binary image. The red line is the computed orientation of the probe based on the center of the bead and the center of the rod. Images were analyzed at 21 Hz. Movie S3-VERTICAL ANGLE. Fluctuations of the vertical angle (α) between the nanorod and the horizontal plane. Top left: bright field movie of the fluctuating nanorod-bead probe tethered by a DNA molecule and subject to the magnetic field created by a cylindrical magnet. Blue and red lines show where the intensity profiles were taken. Note that the diffraction pattern remains homogeneous along the rod which indicates that α is small and relatively constant. Top right: intensity profiles for the red and blue lines in bright field movie. Bottom: plot of the vertical angle as a function of movie frame. We obtain this angle from the heights of the two positions along the nanorod (indicated by the blue and red lines) and the distance between them (660 nm). See Figure S7 caption for details.
Torsional stress in linear biopolymers such as DNA and chromatin has important consequences for n... more Torsional stress in linear biopolymers such as DNA and chromatin has important consequences for nanoscale biological processes. We have developed a new method to directly measure torque on single molecules. Using a cylindrical magnet, we manipulate a novel probe consisting of a nanorod with a 0.1 μm ferromagnetic segment coupled to a magnetic bead. We achieve controlled introduction of turns into the molecule and precise measurement of torque and molecule extension as a function of the number of turns at low pulling force. We show torque measurement of single DNA molecules and demonstrate for the first time measurements of single chromatin fibers.
Movie S1-PROBE ROTATION. Real time movie of the rotation of the nanorod-bead probe by translation... more Movie S1-PROBE ROTATION. Real time movie of the rotation of the nanorod-bead probe by translation of the stage. The probe was tethered to the glass surface of the capillary tube by a 3 µm DNA molecule and lifted using the cylindrical magnet. Movement of the stage in a path around the projection of the center of the cylindrical magnet allows the nanorod-bead probe to be rotated in a controlled manner. Movie S2-NANOROD-BEAD PROBE. Fluctuations of the nanorod-bead probe under a vertical magnetic field created by a cylindrical magnet. Nanorod-bead probe was tethered to the glass surface by a 3µm DNA molecule. The vertical magnetic field lifts the probe while providing a weak horizontal angular trap. The horizontal angular fluctuations in this case spanned ≈100°. The left side is a bright field movie. The right side is the same movie as a binary image. The red line is the computed orientation of the probe based on the center of the bead and the center of the rod. Images were analyzed at 21 Hz. Movie S3-VERTICAL ANGLE. Fluctuations of the vertical angle (α) between the nanorod and the horizontal plane. Top left: bright field movie of the fluctuating nanorod-bead probe tethered by a DNA molecule and subject to the magnetic field created by a cylindrical magnet. Blue and red lines show where the intensity profiles were taken. Note that the diffraction pattern remains homogeneous along the rod which indicates that α is small and relatively constant. Top right: intensity profiles for the red and blue lines in bright field movie. Bottom: plot of the vertical angle as a function of movie frame. We obtain this angle from the heights of the two positions along the nanorod (indicated by the blue and red lines) and the distance between them (660 nm). See Figure S7 caption for details.
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Papers by Rohit Dewan