There is a major difference between DNA polymerase and RNA polymerase: the RNA polymerase can synthesize a new strand whereas the DNA polymerase can only extend an existing strand. Therefore, to synthesize a DNA molecule, a short RNA molecule (~ 5 - 12 nucleotides) must be synthesize first by a special enzyme. The initiating RNA molecule is known as a primer, and the enzyme is called primase.

In addition to DNA polymerase and primase, DNA replication requires helicase and single strand binding protein (SSB protein). The role of helicase is to unwind the duplex DNA. SSB proteins can bind to both separated strands, preventing them from annealing (reconstitution of double-stranded DNA from single strands).

The replication mechanisms in both bacteria and eukaryotes are similar. However, eukaryotic DNA polymerases do not contain a subunit similar to the E. coli b subunit. They use a separate protein called proliferating cell nuclear antigen (PCNA) to clamp the DNA.
DNA polymerases can extend nucleic acid strands only in the 5' to 3' direction. However, in the direction of a growing fork, only one strand is from 5' to 3'. This strand (the leading strand) can be synthesized continuously. The other strand (the lagging strand), whose 5' to 3' direction is opposite to the movement of a growing fork, should be synthesized discontinuously.