Cells are messy. That was Michelle Wang's reaction in graduate school when she gazed at a muscle cell through a microscope. Wang, who studied physics at Nanjing University in China, yearned to understand the fundamental nature of life. But there was too much happening in the cell. "I felt like I didn't understand what was going on, because the systems were so complex," she says.
So when she learned about techniques to manipulate single molecules, Wang's interest soared. "I wanted to look at biological systems at a very basic, fundamental level, to see how things work, to understand mechanisms," says Wang. "And I became very impressed with what the techniques could do, with manipulating molecules and monitoring their motion."
Wang convinced her postdoctoral adviser at Princeton to let her study one of the most fundamental of all cellular processes—DNA transcription. During transcription, DNA is copied into RNA, which later guides the construction of a protein. In the late 1990s, the mechanics of the process were not well understood. So, building on an instrument called optical tweezers that physicists use to manipulate molecules, Wang and her coworkers created a technique to measure exactly how the enzyme that drives transcription, RNA polymerase, marches up and down the DNA double helix. Wang says understanding that motion was like making a movie of transcription in action.
Her two Science papers describing these experiments are now considered classics in the field of biophysics. "We had a feeling they were going to be high-impact papers," says Wang. "It was the first time anybody had looked at the mechanical forces driving the movement of a single DNA-based molecular motor, in this case RNA polymerase."
Soon after, Wang landed a tenure-track position and her own lab at Cornell University. There, she set about refining the techniques to manipulate the suite of enzymes and proteins that constantly read DNA and maintain it and pack it tight in the cell nucleus. But, says Wang, "when I came to Cornell I felt like we simply did not have the tools necessary to probe as deeply as I wanted to."
So Wang and her lab members invented the tools—two of them, in fact, both variations on the optical tweezers. The first allows her to locate exactly where along a stretch of DNA a protein is clamped on. The technique also measures how tightly the protein is attached to the DNA. "It's a very precise technique," says Wang. "We can use it to study all kinds of proteins bound to DNA."
Wang's second innovation adds to the kinds of measurements the optical tweezers can make. Optical tweezers use a laser shot down a microscope to immobilize tiny particles, such as microscopic plastic beads. By attaching a DNA molecule to the plastic bead, researchers can measure the force and position of other molecules, such as proteins, that are attached to the DNA.
Optical tweezers were invented in the 1970s, and Wang's refinements of that tool have allowed her to answer many of the questions that intrigue her. But conventional optical tweezers measure only straight-line forces. That is, they measure how much a segment of DNA or a protein attached to it might get pulled along an axis. "This was not satisfying to me," says Wang. "If you look at a cell, it's a very dynamic place—there's a lot of motion. Not only is there translocation, or straight-line movement, there's also a lot of rotation going on." Because of the double-helical nature of DNA, any protein or enzyme that travels its length will, by necessity, also spin or twist. "That's just the basic geometry of it," says Wang. She wanted to measure this torsion—something no one had accomplished with optical tweezers.
To do so, Wang and her students invented a very clever nanoparticle—an almost impossibly small piece of quartz shaped like a thimble. Optical tweezers can trap these thimbles, just as the standard tiny plastic beads are trapped. And, as with the beads, DNA molecules can be attached to one end of the thimble. But here's the twist: When polarized light shines on the thimble and rotates, the light drags the thimble with it. Wang can spin the thimble inside the optical tweezers and measure the torque, or rotational force, needed to spin the whole thimble-DNA-protein complex.
Wang calls it "really, really cool. Now we can monitor biological processes at the single-molecule level both translationally and rotationally."
Wang expects the system to help illuminate the workings of many cellular processes, such as how structures called DNA supercoils form. "Lots of people have already told me they're interested in using this technique," says Wang. "I expect there will be a lot of work based on it."
And someday soon, once she fathoms the dynamics of these basic cellular processes, Wang wants to zoom out and reexamine whole, messy cells. "Right now, I'm working at a very, very basic physical level," says Wang. "But I realize there are a lot of big biological questions out there that are more than just basic. I want to move up a few levels and understand how a whole cell works."