Taq DNA Polymerase is a highly heat-stable DNA polymerase derived from the thermophilic bacterium Thermus aquaticus. In addition to polymerase activity, this DNA polymerase exhibits 5'-3' exonuclease activity, unlikely to have 3'-5' exonuclease activity due to absence of a 3'-5' exonuclease domain. In molecular cloning Taq DNA polymerase can be used for DNA sequencing and in vitro amplification of specific segments of DNA using the polymerase chain reaction （PCR）. During PCR, Taq DNA polymerase is not inactivated during the denaturation step （~94°C） and can go directly to the second cycle, thus eliminating the need to reintroduce new enzyme at each cycle, making Taq DNA polymerase a unique enzyme for PCR reactions.