<P> One of Taq's drawbacks is its lack of 3' to 5' exonuclease proofreading activity resulting in relatively low replication fidelity . Originally its error rate was measured at about 1 in 9,000 nucleotides . The remaining two domains act in coordination, via coupled domain motion . Some thermostable DNA polymerases have been isolated from other thermophilic bacteria and archaea, such as Pfu DNA polymerase, possessing a proofreading activity, and are being used instead of (or in combination with) Taq for high - fidelity amplification . </P> <P> Taq makes DNA products that have A (adenine) overhangs at their 3' ends . This may be useful in TA cloning, whereby a cloning vector (such as a plasmid) that has a T (thymine) 3' overhang is used, which complements with the A overhang of the PCR product, thus enabling ligation of the PCR product into the plasmid vector . </P> <P> In the early 1980s, Kary Mullis was working at Cetus Corporation on the application of synthetic DNAs to biotechnology . He was familiar with the use of DNA oligonucleotides as probes for binding to target DNA strands, as well as their use as primers for DNA sequencing and cDNA synthesis . In 1983, he began using two primers, one to hybridize to each strand of a target DNA, and adding DNA polymerase to the reaction . This led to exponential DNA replication, greatly amplifying the amounts of DNA between the primers . </P> <P> However, after each round of replication the mixture needs to be heated above 90 ° C to denature the newly formed DNA, allowing the strands to separate and act as templates in the next round of amplification . This heating step also inactivates the DNA polymerase that was in use before the discovery of Taq polymerase, the Klenow fragment of the DNA polymerase I from E. coli . </P>

What is the role of dna polymerase in pcr