37. A Question about Replication Answered
Michelle Wang
Michelle D. Wang, Physics, and a collaborator determined how helicases, a class of enzymes, play a key role in gene copying. The researchers answered a fundamental question about how two strands of DNA—the familiar double helix—separate to start replication, the process by which genes copy themselves. Scientists have known that helicases bind to the area of a double helix where the two strands fork away from each other. The forked area opens and closes very rapidly. Scientists have debated whether helicases actively separate the two strands at the fork or if they passively wait for the fork to widen on its own. Wang and her colleague found that the helicase exerts a force on the fork to separate the two strands—so the mechanism is an active unwinding motor, not a simple passive unwinding. To observe this mechanism in action, they anchored one end of one of the double-helix strands to the surface of a microscope cover slip, while the end of the other strand was attached to a micron-sized plastic bead. They focused a laser beam on the tiny bead, trapping the bead within the beam of light. This setup allowed them to measure the position and force on the bead, creating a very precise sensor of helicase motion. As the helicase moved toward the fork and the double helix unwound, the tension on the two strands lessened. Using statistical mechanics models, the researchers could compare actual measurements of movement with predictions based on both active and passive scenarios. The researchers believe that helicases work with other enzymes, with accessory proteins assisting the helicase by destabilizing the fork junction.