Figure 1

A cartoon model of leading and lagging strand DNA synthesis by the Bacteriophage T4 Replisome. The replicase proteins include the gp43 DNA polymerase, responsible for leading and lagging strand synthesis, the gp45 clamp, the ring shaped processivity factor involved in polymerase fidelity, and gp44/62 clamp loader, an AAA + ATPase responsible for opening gp45 for placement and removal on duplex DNA. The primosomal proteins include the gp41 helicase, a hexameric 5' to 3' ATP dependent DNA helicase, the gp61 primase, a DNA dependent RNA polymerase responsible for synthesis of primers for lagging strand synthesis, the gp32 single stranded DNA binding protein, responsible for protection of single stranded DNA created by gp41 helicase activity, and the gp59 helicase loading protein, responsible for the loading of gp41 helicase onto gp32 protected ssDNA. Repair of Okazaki fragments is accomplished by the RNase H, a 5' to 3' exonuclease, and gp30 ligase, the ATP dependent DNA ligase. Leading and lagging strand synthesis is coordinated by the replisome. Lagging strand primer extension and helicase progression lead to the formation of a loop of DNA extending from the replisome as proposed in the "trombone" model [21].