Every time a human or bacterial cell divides it first must copy its DNA. Specialized proteins unzip the intertwined DNA strands while others follow and build new strands, using the originals as templates. Whenever these proteins encounter a break – and there are many – they stop and retreat, allowing a new cast of molecular players to enter the scene. Scientists have long sought to understand how one of these players, a repair protein known as RecA in bacterial cells, helps broken DNA find a way to bridge the gap. They knew that RecA guided a broken DNA strand to...