Accordingly only RNA polymerases are able to synthesize a polynucleotide de novo . But DNA polymerases are able to add deoxy-mononucleotides either to a DNA or a RNA primer. Consequently DNA polymerases use RNA primers to initialize new DNA strands. Before a cell is ready to devide into two daughter cells, the genom has to be duplicated. The replication of the DNA double helix is a complex process and several proteins are involved.
Replication starts with the unwinding of the double helix and the formation of a replication fork.
As DNA polymerase read their template strands only in 3' to 5' direction, one template strand has to be read against the direction of movement of the replication fork. This strand is the template for the lagging strand. The lagging strand is synthesized within fragments which are called Okazaki fragments and are 1000 to 2000 bases in length. The synthesis of each Okasaki fragment requires a primer. In E. coli the primers are RNA oligonucleotides which are synthesized by a special RNA polymerase called DNA primase.