The replisome is a central element of any replication system and is responsible for the separation and replication of the parent DNA strands. The T4 bacteriophage system offers a simple model for replication. The replisome is formed by the primosome (the helicase (gp41) and primase (gp61) complex) and two holoenzymes (the polymerases (gp43) and their accessory proteins). Many questions remain concerning these complexes, including how the helicase, primase and polymerase activities and motions are coupled. Here we use magnetic tweezers to manipulate a single tethered DNA hairpin. The substrate extension is used as a real-time reporter of the replisome-activity. We have studied the coupling between the helicase and polymerase. We have also investigated how the UvsW helicase can restart a stalled replication fork. Finally, starting from this fundamental research we have design a new sequencing strategy.