The polymerase chain reaction (PCR) is a laboratory technique that makes many identical copies of a chosen segment of DNA by repeatedly cycling a reaction mixture through three temperatures. Starting from a tiny amount of template, PCR can produce billions of copies of a target sequence within a couple of hours, making otherwise undetectable amounts of DNA easy to measure. This article explains the chemistry and thermal physics that drive the amplification. It is a methods explainer about the technique itself and does not address how any result should be interpreted.
The ingredients of the reaction
A PCR mixture contains a few essential components. The template is the DNA sample that includes the region of interest. Two short synthetic DNA fragments called primers are designed to bind to the sequences flanking the target, one on each strand, defining exactly which region will be copied. A heat-stable enzyme, DNA polymerase, carries out the copying, and a supply of the four DNA building blocks, the deoxynucleotide triphosphates, provides the raw material. A buffer with magnesium ions creates the chemical conditions the enzyme needs.
The enzyme is what made modern PCR practical. Early polymerases were destroyed by the high temperatures the method requires. Taq polymerase, isolated from a bacterium that lives in hot springs, remains active after repeated heating, so it survives every cycle and need not be replenished. Its thermal stability is the property that allows the reaction to be automated.
Thermal cycling: the three steps
PCR proceeds by repeating a cycle of three temperature stages, each driving a distinct molecular event.
| Step | Approximate temperature regime | What happens |
|---|---|---|
| Denaturation | High | Heat separates the double helix into two single strands |
| Annealing | Lower | Primers bind to their matching flanking sequences |
| Extension | Intermediate | Polymerase builds a new complementary strand from each primer |
In denaturation, raising the temperature breaks the hydrogen bonds holding the two DNA strands together, so the double helix unzips into single strands. In annealing, cooling allows the primers to find and bind their complementary sequences; the annealing temperature is chosen to match the primers so they bind specifically and not elsewhere. In extension, the polymerase starts at each bound primer and adds nucleotides one by one, reading the template and building a matching new strand. A machine called a thermal cycler moves the tubes through these temperatures automatically and reproducibly.
Why amplification is exponential
The power of PCR comes from repetition. After the first cycle, each original target region has become two copies. In the second cycle each of those is copied again, giving four, then eight, and so on. Because every product of one cycle becomes a template for the next, the number of copies roughly doubles each cycle, growing exponentially. After around thirty cycles a single starting molecule can in principle yield on the order of a billion copies. This exponential growth is what turns a trace amount of DNA into a quantity large enough to measure by other means.
The technique sits in the same family of physical and chemical measurement methods as mass spectrometry, in that both convert a property of a sample into a measurable signal under tightly controlled conditions.
What real-time and quantitative PCR add
Standard PCR tells you how much product accumulated by the end. Real-time PCR, also called quantitative PCR or qPCR, measures the amount of product as the reaction proceeds, cycle by cycle. It does this by including a fluorescent reporter whose signal increases as more DNA is made; an optical system reads the fluorescence after every cycle. The cycle at which the signal crosses a set threshold relates to how much target was present at the start: more starting material crosses the threshold sooner. This turns PCR from a yes-or-no copying reaction into a quantitative measurement, provided the assay is properly calibrated. Reverse-transcription PCR extends the method to RNA targets by first copying RNA into DNA.
Reporting a PCR method
Reproducibility depends on documenting primer sequences, cycle temperatures and times, the polymerase used and the cycle number, exactly the kind of parameter recording covered in our guide on reporting analytical methods reproducibly. Standard terminology lives in the CASRAI dictionary, and the broader context of measurement in research appears across our research lifecycle articles.
Frequently asked questions
What do the primers determine?
The primers define precisely which DNA region is copied. Because the polymerase can only extend from a bound primer, only the sequence between the two primers is amplified. Designing primers to match unique flanking sequences is what gives PCR its specificity.
Why is a heat-stable polymerase necessary?
Each cycle includes a high-temperature denaturation step that would inactivate an ordinary enzyme. A heat-stable polymerase such as Taq survives repeated heating, so the same enzyme works through every cycle without needing to be added again, which is what allows the reaction to be automated.
What makes the amplification exponential?
Every newly made strand becomes a template in the next cycle, so the copy count roughly doubles each round. Repeated doubling produces exponential growth, turning a single molecule into billions of copies over roughly thirty cycles.
What does quantitative PCR measure that standard PCR does not?
Quantitative PCR tracks the amount of product in real time using a fluorescent signal, so the cycle at which the signal crosses a threshold reports how much target was present initially. The reproducibility of such quantification is discussed in our reproducibility coverage and the author guidance.
