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T4 DNA polymerase catalyzes the synthesis of DNA in the 5′→3′ direction and requires the presence of template and primer. This enzyme possesses 3′→5′ exonuclease activity, which is much higher than that found in DNA polymerase I （E. coli）. Unlike E. coli DNA polymerase I, T4 DNA polymerase lacks 5’ →3’ exonuclease activity. T4 DNA polymerase can be used to Removal of 3’ overhangs or fill-in of 5’ overhangs to form blunt ends.