The invention proposes a device for analyzing nucleic acid molecules (M), comprising: —a bead (20), on which one molecule can be anchored at one end, —a surface (520), on which the molecule can be anchored at the other end, —an actuator (30), adapted to cause the bead to move relative to said surface in one direction, —a sensor (50), adapted to measure a distance between the bead and the surface, the device further comprising a well (11), having an axis (X-X) extending along the direction of motion of the bead and a bottom (110) formed by said surface, said well being filled with electrically conductive solution (40), and receiving the bead, the sensor being adapted to measure an impedance of the well, depending on a distance between the bead and the surface, to determine, the distance between the bead and the surface.