The Kornberg enzyme, now called polymerase I, is no longer considered to be an essential enzyme for DNA synthesis. Polymerase I is a single polypeptide chin with a molecular weight of 109,000. Thus DNA polymerases are metalloenzymes.
Polymerase III consists of two polypeptide chains having molecular weights of 140,000 and 40,000. Like polymerase II it has 5'-3' polymerization activity and 3'-5' exonuclease activity. Recent work has shown that polymerase III is the main polymerizing enzyme. It can polymerize about 15,000 nucleotides per minute in E.coli. Polymerization is the synthesis of a complementary DNA strand from deoxyribonucleoside triphosphate monomers.