text: (a) Schematic drawing of the magnetic tweezers. A DNA molecule is attached at one end to the bottom of the flow cell and at the other end to a magnetic bead. This molecule can be pulled and twisted using small magnets placed above the flow cell. The position of the magnetic bead is measured using an inverted microscope placed beneath the flow cell. The bead position and thus the end-to-end distance of the DNA molecule is determined using video microscopy and image analysis. (b) Extension of a DNA molecule versus the number of turns applied by the magnets for various stretching forces. At low force, contraction of the molecule is symmetrical under positive and negative applied turns. At higher force, the molecule's extension initially remains constant for positive applied turns. The induced torque increases linearly with the number of applied turns, as depicted in the top graph until a buckling transition allows the system to saturate its torsional constraint through the formation of plectonemes.
  
entities.text: ['cell']
entities.type: Cell
entities.entity_id: PMC-1075924-caption-01_T1
entities.text: ['cell']
entities.type: Cell
entities.entity_id: PMC-1075924-caption-01_T2
entities.text: ['cell']
entities.type: Cell
entities.entity_id: PMC-1075924-caption-01_T3
entities.text: ['plectonemes']
entities.type: Cellular_component
entities.entity_id: PMC-1075924-caption-01_T4