DNA polymerize requires the four dNTP's and Mg2+ to synthesize DNA. The reaction is thermodynamically favorable because it is necessarily accompanied by cleavage of pyrophosphate. This means that DNA is not degraded in the reverse reaction by the same enzymes that catalyze its formation. Interesting!

DNA polymerase cannot begin a new chain from scratch without a primer (which is synthesized by primase). There must be a preexisting 3'-OH group. Primers consist of RNA and DNA but always begin with RNA.