The trombone model of replication fork function. The lagging-strand polymerase extends the 3′ terminus of an Okazaki fragment in the opposite direction to fork movement, yet this polymerase is part of the replisome and thus moves with the fork. The opposed directions result in formation of a DNA loop for each Okazaki fragment. As multiple Okazaki fragments are synthesized, loops repeatedly grow and are released, similar to the movement of a trombone slide as the instrument is played. For simplicity, the participation of the third Pol III core in lagging-strand synthesis is not shown here.