adaptation for studies of the DNA molecule, and even of bacteria and other cells.

A point of departure for this work lies in the fact that micron-size polystyrene spheres can respond to a single focused laser in the same fashion as atoms. These spheres also redistribute their internal electrons in response to the rapidly varying electric field of the laser and then move in the direction of the focus of its light, where its intensity is the strongest. As with atoms, the rule in choosing a laser frequency is simply to avoid one at which the spheres absorb light strongly. But these spheres, unlike atoms, are large enough to be visible through a light microscope. This makes it possible to integrate such a laser with this microscope and to observe the particles while manipulating them.

The basic idea, that of using a single laser beam for this purpose, is that of Arthur Ashkin, of AT&T Bell Laboratories. Chu and his colleagues call it "optical tweezers" and have been using it in studies of DNA. The DNA in a human cell is a meter in total length, yet it coils up to fit neatly within the cell's nucleus. Chu's group has begun its studies of DNA by attaching tiny spheres to each end of a strand of DNA, stretching it out to full length using a pair of optical tweezers. By measuring the force required to stretch the DNA out to a given length, they are able to compare their results to basic models of polymer elasticity. In addition, with this new-found method of manipulating the molecule, they intend to study the function of enzymes that act on the DNA.

They also have spot welded such spheres to a microscope slide, by increasing the laser power, leaving the DNA fixed to the slide and available for further research. Chu notes that these could include studies of the interaction of enzymes with DNA, including those involved with the expression of genes and with the editing out of DNA errors, which could permit the DNA to mutate into genetic nonsense. In another study of the mechanical properties of large biological molecules, Chu's group has been studying the contraction of muscle at a fundamental level. The pertinent molecule is called myosin, and these investigators have been using optical tweezers to study the force of its contraction.

Other investigators have used such tweezers to manipulate cellular organelles. Ashkin and his colleague J. B. Dziedzic have made the surprising discovery that optical tweezers can handle live bacteria and other cells without damaging them. This is possible because the cells are nearly transparent to the laser and are immersed in water, which offers an effective coolant. Indeed, Ashkin has manipulated objects within a cell without puncturing the cell wall. In a potentially important application,



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