Tortoise puts brake on hare

The leading strand DNA polymerase keeps going straight forever. But the lagging strand DNA polymerase hops off the DNA, finds the new RNA primer made by DNA primase, then gets going again. Jong-Bong Lee, Antoine van Oijen (Harvard Medical School, Boston, MA), and colleagues now show that DNA primase acts as a brake for the leading strand DNA polymerase, so that the two polymerases stay in synch.

Even these “incredibly simple systems,” says van Oijen, “have transient intermediates with no product, for example these pauses, so you need single-molecule studies in real time.” But even simple DNA polymerase systems are multimeric complexes, whose in vitro assembly yields a few percent of productive complexes.

Rather than do hundreds of individual laser trap experiments, the Boston team stuck one end of the DNA...