FEMTONEWTON FORCE MEASUREMENTS OF DNA MOLECULES are carried out by Caltech physicists Jens-Christian Meiners and Stephen Quake (626-395-3362, quake@caletch.edu) by attaching beads at either end and holding them in separate "optical tweezers," focused laser beams that trap the beads with radiation pressure (see figure at Physics News Graphics).

The resulting graph of the cross-correlations between the beads constitutes a sort of force spectrum, from which mechanical and dynamic properties can be deduced. The new Caltech advance is to make such measurements with femtonewton precision, a hundred times better than previous efforts. And all of this is accomplished in a wet, warm environment typical of biology, not in a chilled vacuum.

With the new force resolving power, several longstanding problems in polymers dynamics could be solved. For example, contrary to expectation Meiners and Quake showed that the relaxation time of the molecule actually decreases with greater extension. They also showed that an extended polymer is hydrodynamically equivalent to a rigid rod. (Meiners and Quake, Physical Review Letters, 22 May; Select Article.)