The enzyme that synthesizes RNA primers during DNA replication is primase.
Primase is a specialized RNA polymerase that builds a short RNA segment complementary to the DNA template. This primer supplies the free 3′-hydroxyl group that DNA polymerase needs before it can begin adding DNA nucleotides. Without primase, the main DNA-synthesizing enzymes cannot start a new strand from scratch.
Primase is particularly important on the lagging strand, where DNA is made discontinuously as Okazaki fragments. It lays down repeated primers, and DNA polymerase extends each one. Later, the RNA primers are removed, the gaps are filled with DNA, and DNA ligase seals the remaining breaks in the sugar-phosphate backbone.
In bacteria such as Escherichia coli, the DnaG primase typically makes RNA primers about 11 nucleotides long. In eukaryotes, primase works as part of the DNA polymerase alpha–primase complex. A frequent mix-up is assigning this job to helicase, polymerase, or ligase: helicase unwinds DNA, polymerase extends primers, and ligase joins fragments.