PCR
PCR, or polymerase chain reaction, is a laboratory technique for rapidly amplifying a selected DNA segment. By cycling temperature and using primers plus a heat-stable DNA polymerase, PCR can turn tiny amounts of genetic material into enough copies for analysis.
What PCR is
PCR is a technique for making many copies of a chosen DNA segment. It does not copy an entire genome by default. Instead, researchers design primers that define the target region, then use repeated temperature cycles to amplify that region from a DNA sample.
What goes into the reaction
A basic PCR mix includes template DNA, two primers, DNA nucleotides, buffer, magnesium ions, and a DNA polymerase that can survive heating. The primers mark the start and end of the sequence to be copied, while the polymerase builds new DNA strands.
Denaturation
Each cycle begins with denaturation, a high-temperature step that separates double-stranded DNA into single strands. This exposes the bases so primers can bind in the next step. Heat-stable polymerases made PCR practical because they remain active after repeated heating.
Annealing
During annealing, the reaction cools so primers can bind to complementary sequences on the template DNA. The temperature must be chosen carefully: too high and primers may not bind; too low and they may bind to the wrong places.
Extension
During extension, the DNA polymerase adds nucleotides from each primer's 3-prime end, copying the target region. Repeating the cycle can double the number of target copies each round in ideal conditions, producing large amounts of DNA from a small starting sample.
Why primers matter
PCR specificity depends heavily on primer design. Primers choose which DNA segment gets amplified, and their sequence, length, melting temperature, and placement affect success. Poor primers can cause weak amplification, nonspecific products, or primer-dimer artifacts.
PCR variants
Many PCR variants adapt the core idea. RT-PCR first converts RNA into complementary DNA. Quantitative PCR tracks amplification in real time. Digital PCR partitions a sample to count molecules more precisely. Multiplex PCR amplifies multiple targets in one reaction.
Uses
PCR is used in medical diagnostics, pathogen detection, genetic testing, forensic analysis, ancestry research, cloning, sequencing preparation, environmental DNA surveys, ancient DNA work, and basic biology. Its power comes from making rare target sequences detectable.
Why it matters
PCR made DNA analysis faster, cheaper, and more sensitive. It lets scientists and clinicians work with samples too small to study directly, which transformed molecular biology, public health testing, forensics, conservation, and biotechnology.