Taq polymerase lacks a 3' to 5' exonuclease activity which means it is unable to proof-read the base just added. If the base is not complementary to the corresponding base it is not possible for it to be removed and replaced as is the case with other DNA polymerases. Thus Taq polymerase has an error rate of about 1 in 10,000 bases. If an error occurs early in the reaction then a significant amount of the final product will have the mistake.
For accuracy other polymerases are now available for use in vital cases such as amplification for sequencing and DNA profiling. Examples of polymerases with 3' to 5' exonuclease activity include:
- Pfu DNA polymerase, extracted from Pyrococcus furiosus;
- Vent, which is extracted from Thermococcus litoralis;
- Pwo, which is extracted from Pyrococcus woesii.
These enzymes may work at a slower rate and so more time must be allowed for the elongation phase of the cycle.