Initiation of DNA synthesis requires an RNA primer. DNA polymerases cannot initiate DNA chains de novo. (RNA polymerase, in contrast, can initiate chains de novo during trascription). A pre-existing polynucleotide chain (the primer) is required to which polynucleotides are added. The primer RNA is a short polynucleotide chain with a 5' triphosphate end and a 3'-OH end. The RNA primer is synthesized by the DNA template close to the origin of replication. This synthesis is probably catalysed by a modified RNA polymerase which may contain a starting factor. In E. coli RNA polymerase is required just before replication of DNA, but protein synthesis is not.