This diagram shows the synthesis of an Okazaki fragment on the lagging strand; synthesis on the leading strand is not shown but involves the same steps. (a) Helicase unwinds the DNA helix, and the resulting single-stranded DNA is coated with RPA proteins. (b) The polymerase alpha–primase synthesizes a short RNA–DNA primer in the 5' to 3' direction, displacing RPA molecules as it travels along the template. (c) Following completion of the primer, the clamp loader displaces polymerase alpha and catalyzes loading of the sliding clamp and DNA polymerase delta or epsilon. (d) The polymerase machine extends the primer until it reaches the 5' end of the previous Okazaki fragment. (e) The primer of the downstream Okazaki fragment is removed by a nuclease (not shown), allowing DNA polymerase to continue along the template to the 5' end of the next DNA fragment. (f) After completion of the new DNA strand, the nick between the two fragments is sealed by DNA ligase.^[1]