Twisting and Stretching
Single Molecules
Adding a twist to DNA


Single Molecule experiments have become a new field of research in the 1990 years with the appearing of new techniques allowing the manipulation and visualization of single molecules together with strong biophysical motivations. Biologists were interested in the understanding of mechanism involved in proteins such as molecular motors,  the usual test tube experiments used so far, lead only to averaged results, whereas the X-ray crystal structure are tremendous snap-shot of a freeze  emzymatic configuration. Observing the power stroke of a single molecular motor was a gedanken experiment until it became a reality in S.M. Block and J. Spudich group. These works where followed by the impressive demonstration by the Kinosta group that a single enzyme the F1-ATPase was indeed a rotary motor.

Means to visualize

Molecular motors and enzymes in general are nanometer scale objects, moreover their natural environement is basically water. Unfortunately, the only microscope that operates in water is the usual optical one with a resolution dictated by the wavelength of light (500nm) which cannot resolve those object. Most single molecule experiments use a marker attached to the enzyme which will be the object seen by the optical microscope. Typically this marker is a micron-size bead. Nanometer scale objects move also by a few nanometers, detecting those motions thus requires to detect the minute displacements of the marker. The good news is that this is possible with the optical microscope. This may sound surprising in view of the limitation in resolution that we just mentionned, the simple answer to this paradox is that the resolution limitation does not apply to the displacement of the object. One cannot distinguish two 100 nm beads sitting side by side but one can detect a 1nm displacement of such a bead.

Why do we need to pull

If the ability to measure accurately the position of small objects is an obvious desirable feature in single molecule experiment, the ability to apply a force on them is less intuitive. At the molecule scale, everything is fluctuating like hell, this is the so called brownian motion. An enzyme binding a DNA molecule is moving all over the place if the DNA molecule is not stretched. To measure the displacement of an enzyme or of a molecular motor, it is fundamental to immobilize its substrate either by glueing it or by applying a force on it. The micromanipulation techniques that we describe below all use different manners to apply such a force on a bio-polymer. Not only do we need to apply a force but also we must often measure the magnitude of this force. This turns out to be the most difficult part of the micromanipulation techniques.

K. Svoboda, CF. Schmidt, BJ. Schnapp, SM. Block,
Nature (1993) 365-6448 p.721 PubMed CrosRef
JT. Finer, RM. Simmons, JA. Spudich,
Nature (1994) 368-6467 p.113 PubMed CrosRef
3Direct observation of the rotation of F-1-ATPase
H. Noji, R. Yasuda, M. Yoshida, K. Kinosita,
Nature (1997) 386-6622 p.299 PubMed CrosRef
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