Activities required at a DNA replication fork. (a) DNA polymerase and the β sliding clamp are required for processive DNA synthesis. (b) Helicases that function at replication forks are hexamers that encircle single-stranded DNA and translocate on the DNA strand to separate the strands of the parental duplex. (c) A topoisomerase removes twists in the DNA. In E. coli, the topoisomerase is DNA gyrase. (d) Primase (an RNA polymerase) makes short RNA primers to initiate DNA synthesis. Primases typically bind the helicase, thus localizing primers to the replication fork. (e) Ligase seals DNA nicks, joining Okazaki fragments together (after removal of the RNA primers). (f) Single-stranded DNA–binding protein (SSB) binds cooperatively to single-stranded DNA (ssDNA), removing secondary structure in the DNA strand and protecting it from the action of nucleases.