text: why do we often use taq dna polymerase in a pcr?   taq polymerase originated in a thermophile, thermus aquaticus.  at this point, the taq you actually use is probably the taq gene cloned into e. coli.  anyhow, as a thermophile, thermus aquaticus lives in very high temperatures, and its enzymes can also function at the elevated temps.  this is important for pcr, where your extension is at 70c.  the ability to withstand high temps is also important because of the denaturation step which separates your strands and allows for the continued amplification of pcr product.    "back in the day" before thermostable polymerases, each round of amplificatin required the addition of new enzyme.  you can imagine how fun that must have been.  new thermostable enzymes are more thermostable, or faster, or have higher fidelity, etc.  taq, however, for all of its short coming, is still mega cheap and used quite a bit.