PCR Amplification: Steps of Polymerase Chain Reaction

30 September 2026Biology Try the interactive 3D model →

The polymerase chain reaction, or PCR, is a method to make millions of copies of a chosen DNA segment in a test tube. It copies only the region between two short DNA pieces called primers.

PCR matters because often the starting DNA is too little to see or test. In a few hours, PCR turns a trace from a blood sample, a hair root or a microbe into enough DNA for diagnosis, fingerprinting and cloning.

Key Concepts

Primer is a short DNA piece that matches each end of the target region. Taq polymerase is a heat-stable DNA-building enzyme from Thermus aquaticus. Template is the starting DNA. Cycle is one round of heating and cooling. Amplicon is the copied product. Temperatures are commonly about 94-95 degrees Celsius for separation, 50-65 for primer joining, and 72 for building new DNA.

Formulas

Ideal copy growth follows N = N0 x 2 to the power n, where N is final copy number, N0 is starting copies, and n is cycle number. This assumes every molecule is copied each cycle (100 percent efficiency), enough primers and nucleotides, and no inhibitors. Real PCR efficiency is lower, so actual copies are somewhat less, but growth is still extremely fast in early cycles.

Worked Example

A reaction starts with 100 target DNA molecules and runs 25 cycles at ideal efficiency. Step 1: use N = 100 x 2 to the power 25. Step 2: 2 to the power 10 is 1,024 (about 10 to the power 3), 2 to the power 20 is about 10 to the power 6, and 2 to the power 25 = 33,554,432. Step 3: N = 100 x 33,554,432 = 3,355,443,200 copies, about 3.36 billion copies. This shows why even a tiny starting sample becomes easily detectable.

Common Misconceptions

PCR copies the whole genome. It copies only the short segment between the two primers.
More cycles always give a better result. Too many cycles can increase errors and unwanted products when materials run low.
Any polymerase can be used. Normal polymerases are destroyed by heating, so heat-stable Taq polymerase is needed.

Real-World Applications

PCR tests detect infections such as tuberculosis and many viral diseases, identify crop pathogens, amplify DNA for fingerprinting and paternity tests, check cloned genes, and support research on ancient DNA from fossils and preserved remains.

To recap, PCR repeatedly separates DNA strands, joins primers and builds new strands with Taq polymerase. This cycle multiplies one chosen DNA region into billions of useful copies.