If you see two beads (pictured below) we're probably running RNA polymerase experiments. Each of the two beads is held in an optical trap, while a piece of DNA is suspended between them. The left bead is attached to the DNA directly, while the right bead is attached through the polymerase. As the enzyme moves along the DNA, synthesizing RNA, the length of DNA between the beads is shortened. We monitor the enzyme's activity by watching the beads get closer together.
We can measure the length of DNA between the beads by stretching it! As we move the left bead with a steerable optical trap the DNA becomes taught and starts to pull the right bead. Because we know the position and the force of the two optical traps, we can infer the length of DNA. Click here to watch a movie of DNA stretching [Stretch2.avi 735 KB].