Figure 1.

Schematic representation of the measurement configuration and the three overstretching mechanisms. The NA duplex is attached by three of its four single-strand extremities to two microscopic beads (beads and molecule are not at the same scale). The two beads of each dumbbell are captured in separate optical traps (orange). Force versus displacement curves are obtained by measuring the position of one bead within the trap to nanometer precision, whereas the other trap is displaced. Peeling of the free strand, bubble formation, and S-structure formation are presented from top to bottom. Single strands under force and the S-structure exhibit a longer separation between adjacent nucleotides than the regular double helix. Base pairing is maintained in the S-structure, but basepair stacking and the number of helical turns are strongly reduced (32, 33). Our molecular constructs are free to rotate around the axis of applied force as on one side the bead is attached only to a single strand (green strand on the left-hand side of the figure). To see this figure in color, go online.