DNA polymerase is an enzyme that catalyses the polymerisation of deoxyribonucleotide. The enzyme uses magnesium ion as co-factor. DNA polymerase can add free nucleotides to only the 3' end of the newly forming strand. This results in elongation of the new strand in 5' - 3' direction. The enzyme cannot initiate a new chain but it can only add a nucleotide to the pre-existing 3 - OH group and therefore, it needs a primer at which it can add the first nucleotide. Prokaryotes have three major types of DNA synthesising enzymes called DNA polymerases III. II and I. DNA polymerase III is mainly involved in DNA replication. Polymerase I is major repair enzyme. Polymerase II is minor repair enzyme. In eukaryotes five types of DNA polymerases are found - a, b, g, d and e, but the major three are a, d and e. Polymerase d is involved in replication of leading strand. Polymerase e may help in synthesis of lagging strand along with other roles. Polymerase a is largest and main enzyme of replication of DNA.