DNA polymerase III lacks a 5′→3′ exonuclease activity and so stops making DNA when it reaches the RNA primer of the next Okazaki fragment. At this point DNA synthesis is continued by DNA polymerase I, which does have a 5′→3′ exonuclease activity, and which works in conjunction with RNase H to remove the RNA primer and replace it with DNA. DNA polymerase I usually also replaces some of the DNA from the Okazaki fragment before detaching from the template. This leaves a single missing phosphodiester bond, which is synthesized by DNA ligase, completing this step in the replication process.