(A) DNA polymerase III holoenzyme is released from the lagging strand when it collides into the previously synthesized downstream Okazaki fragment. Neighboring Okazaki fragments are thus separated by a nick. The RNA primer (red) portion of an Okazaki fragment is about 10 nucleotides long and the DNA (green) portion is 1,000 to 2,000 nucleotides long. (B) DNA polymerase I now binds to the nick and removes the RNA of the downstream Okazaki fragment by the 5'-to-3' exonuclease while simultaneously extending the DNA (brown) of the upstream Okazaki fragment. This is equivalent to the process of nick translation. The net result is that RNA primer (red) is replaced by DNA (brown). DNA polymerase I only synthesizes about 10 to 20 nucleotides before it falls off of the DNA. This is enough to do the job. (C) The remaining nick must be joined together with DNA ligase. E. coli DNA ligase uses NAD+ as the source of energy. The net result is that Okazaki fragments are joined together as longer, continuous DNA without any RNA sequences.