 The eukaryotic polymerase alpha, Paul-alpha, is responsible for synthesizing an RNA-DNA hybrid primer of 20 to 30 nucleotides. This process involves the actions of four proteins, Paul-1, Paul-12, Primase-1, Pr-1, and Pr-2. Paul-1 and Pr-1 contain the DNA polymerase and RNA primase activities, respectively. It was previously unknown how Paul-alpha transfers the RNA primer from Pr-1 to Paul-1 for DNA primer extension, or how the primer length is determined. Using cryoelectron microscopy, researchers have now observed the movement of these proteins during the different stages of the Paul-alpha reaction cycle. They found that Paul-1 core moves to take over the three end of the RNA from Pr-1, limiting the length of the DNA primer. Additionally, they discovered that the stress caused by this movement is what ultimately determines the length of the primer. This article was authored by Zwaning Yuan, Boxana Jogescu, Weilin Li, and others. We are article.tv, links in the description below.