Taq Polymerase
Definition
Thermostable DNA polymerase from Thermus aquaticus, the enzyme that makes PCR practical. Stable at 95°C, it extends primers in the 5'-to-3' direction but lacks 3'-to-5' proofreading activity, so error rate per cycle is approximately 10⁻⁴.
- Source organism
- Thermus aquaticus
- Thermal stability
- Active at 95 degrees C
- Synthesis direction
- 5' to 3'
- Proofreading
- None (no 3' to 5' exonuclease activity)
- Error rate
- Approximately 10^-4 per cycle
Common questions
Why does Taq's lack of proofreading matter for forensic PCR?+
Without proofreading, Taq can misincorporate a base and carry that error forward through subsequent amplification cycles. For STR typing the target loci and primers are short and well validated, which keeps this risk low in practice, but it is why some diagnostic and sequencing applications use a proofreading polymerase instead.
Why was Taq polymerase such an important discovery for forensic DNA work?+
Before thermostable polymerases, fresh enzyme had to be added after every heat-denaturation step because ordinary polymerases were destroyed by the heat. Taq survives the 95 degree C denaturation step, so PCR could be automated in a single closed thermal cycler, which made routine forensic DNA typing practical.
Related terms
- Amplicon
- The double-stranded DNA fragment produced between two PCR primers. In forensic STR typing, amplicon size determines whether a locus can be recovered...
- Depurination
- Hydrolytic cleavage of the N-glycosidic bond between deoxyribose and a purine base (adenine or guanine), generating an abasic (apurinic) site. Abasic sites...
- Interlocus Drop-Out
- Failure of larger-amplicon loci to amplify in a degraded-DNA sample while shorter loci succeed. Results in a partial profile that includes only...
- Mini-STR
- A PCR assay in which both primers sit within a few base pairs of the STR repeat, producing amplicons as short as...