Taq is the enzyme that builds new DNA strands during PCR. It’s a heat-resistant DNA polymerase originally isolated from Thermus aquaticus, a bacterium that thrives in hot springs. Its job is straightforward: once the two strands of a DNA molecule are pulled apart by heat and short primer sequences attach to the exposed template, Taq reads the template strand and assembles a matching copy, one nucleotide at a time. Without it, the repeated heating cycles that make PCR work would destroy the copying machinery.
How Taq Builds a New DNA Strand
Taq works during the extension phase of each PCR cycle, typically at around 72°C to 75°C. It latches onto the spot where a primer has attached to single-stranded DNA, then moves along the template in one direction (5′ to 3′, in biochemistry shorthand), grabbing free nucleotides floating in the reaction mix and snapping them into place. Each nucleotide is selected because it pairs with the corresponding base on the template: A pairs with T, C pairs with G. The result is a brand-new strand that’s complementary to the original.
At its optimal temperature, Taq adds roughly 1,000 nucleotides per minute, or about 60 base pairs per second. That speed means a typical gene-length target of 1,000 to 2,000 base pairs can be copied in about one to two minutes per cycle. For longer targets, you simply extend the time at 72°C to give Taq enough time to finish.
Why Heat Resistance Matters
PCR depends on cycling between extreme temperatures. During the denaturation step, the reaction is heated to around 95°C to separate the two strands of DNA. Most enzymes would unfold and stop working at that temperature. Taq survives because it evolved in a microorganism that lives in near-boiling water, so its protein structure holds together through repeated heating. This thermostability is the entire reason Taq revolutionized PCR. Before Taq, researchers had to add fresh enzyme after every denaturation step, making the process slow and labor-intensive.
Taq does gradually lose activity over many cycles at 95°C, but it retains enough function for the 25 to 35 cycles a standard PCR reaction requires.
What Taq Does at Each PCR Step
A single PCR cycle has three temperature stages, and Taq plays a different role in each:
- Denaturation (about 95°C): The high heat separates double-stranded DNA into two single strands. Taq is mostly inactive here but survives the temperature.
- Annealing (about 50°C to 65°C): Short primer sequences bind to complementary regions on the single-stranded template. Taq may begin low-level synthesis at this stage, though the temperature is below its optimum.
- Extension (about 72°C to 75°C): This is Taq’s main working phase. It extends from each primer, reading the template and building new DNA until the strand is complete or the cycle moves on.
After each cycle, the number of target DNA copies roughly doubles. Over 30 cycles, a single starting molecule can theoretically become over a billion copies.
What Taq Needs to Work
Taq can’t function on its own. It requires magnesium ions as a cofactor, typically at a concentration of 1.5 to 2.0 millimolar in the reaction mix. Magnesium helps the enzyme grip the nucleotide building blocks and catalyze the bond-forming reaction. Too little magnesium and the reaction produces nothing. Too much and Taq becomes sloppy, copying non-target sequences and generating unwanted products.
The reaction also needs a supply of the four DNA nucleotide building blocks (collectively called dNTPs), the template DNA you want to copy, and the primers that tell Taq where to start.
Taq’s Error Rate
Taq is fast but not perfectly accurate. It makes roughly one mistake per 100,000 nucleotides it copies per cycle. That sounds rare, but errors accumulate over many cycles because each mistake gets copied forward into subsequent rounds. For routine applications like detecting whether a specific gene is present, this error rate is perfectly acceptable. For work where the exact sequence matters, such as cloning a gene for protein production, researchers often use higher-fidelity polymerases that have built-in proofreading ability. Taq lacks this proofreading function, which is the main reason for its relatively higher error rate.
The A-Overhang Quirk
Taq has a habit that turns out to be useful: after it finishes copying a strand, it tacks on an extra adenine (A) nucleotide at the end, even though the template doesn’t call for one. This leaves the finished PCR product with single-base A-overhangs on both ends. Researchers exploit this in a cloning technique called TA cloning, where the PCR product is inserted directly into a vector that has matching T-overhangs. The A and T ends pair up, making ligation straightforward. If you’re using a polymerase that produces blunt ends instead, TA cloning won’t work without an extra processing step.
Hot-Start Versions of Taq
One drawback of standard Taq is that it has some activity even at room temperature, before the PCR cycling begins. While you’re setting up the reaction on your bench, Taq can start extending primers that have loosely and incorrectly bound to non-target sequences, leading to unwanted background products. Hot-start versions solve this by keeping Taq inactive until the first high-temperature denaturation step. This is achieved in several ways: antibodies that block the enzyme’s active site and release at high heat, chemical modifications that require a 95°C activation step of 10 to 20 minutes, or wax barriers that physically separate the enzyme from the rest of the reaction until the tube is heated. The result is cleaner, more specific amplification.