Molecular diagnostics · BEGINNER LESSON
PCR
Polymerase chain reaction (PCR) is a molecular testing method that copies a selected nucleic acid sequence until it can be detected. It uses primers, a polymerase enzyme, and repeated heating and cooling cycles to make many copies from a small starting amount.
What PCR is and what it measures
PCR is a molecular test used to detect and amplify a chosen nucleic acid target, usually DNA, from a biological sample. In practice, the test measures how much target was present at the start by tracking the amount of copied product that appears after repeated cycles; in real-time PCR, fluorescence can be followed during the reaction, and the cycle number at which fluorescence becomes detectable relates to the starting amount. [PCR = polymerase chain reaction; nucleic acid = genetic material such as DNA or RNA.]
[1] Molecular Diagnostics of Infectious Diseases - PMC - NIH [2] RAZOR: a database of PCR primers targeting human ... - PMC [3] Overview of PCR-based Diagnostic Assays for Emerging ...
How PCR works
The basic workflow has three parts: nucleic acid extraction and purification, amplification, and detection. During amplification, the reaction goes through repeated temperature steps: denaturation separates the strands, annealing lets short PCR primers bind to matching target regions, and extension lets a DNA polymerase enzyme copy the target. The cycle is repeated dozens of times, so the target sequence is copied again and again. In reverse transcription PCR (RT-PCR), reverse transcriptase first converts RNA into complementary DNA (cDNA) before PCR starts.
[1] Molecular Diagnostics of Infectious Diseases - PMC - NIH [2] RAZOR: a database of PCR primers targeting human ... - PMC
Ways the product is detected
PCR products can be detected at the end with gel electrophoresis, which separates DNA fragments so the amplified band can be seen. Real-time quantitative PCR (qPCR) uses fluorescent probes or dye-based detection to monitor amplification during the run. High-resolution melting (HRM) can also confirm the target by watching fluorescence change as double-stranded DNA separates into single strands when heated.
[1] Molecular Diagnostics of Infectious Diseases - PMC - NIH [2] RAZOR: a database of PCR primers targeting human ... - PMC
Why primer design matters and where PCR is used
PCR primers are short nucleotide sequences that bind only to chosen target regions, so primer design is a critical step in making the assay specific. PCR-based tests are widely used in clinical and field settings because they can be fast, sensitive, specific, and practical for known targets. Related designs include multiplex PCR, which detects multiple targets at once, and pan-PCR, which uses conserved gene regions to screen unknown pathogens.
[2] RAZOR: a database of PCR primers targeting human ... - PMC [3] Overview of PCR-based Diagnostic Assays for Emerging ...
Limitations and trade-offs
PCR depends on thermal cycling, so the reaction needs equipment that can repeatedly heat and cool the mixture, and the time to move between temperatures affects total run time. Real-time detection links fluorescence timing to starting target amount, but final fluorescence intensity alone may not reflect the original amount well. Some related methods avoid cycling: loop-mediated isothermal amplification (LAMP) and recombinase polymerase amplification (RPA) can work at a constant temperature, but each has its own limits such as false positives, nonspecific amplification, or quantification difficulty.
[1] Molecular Diagnostics of Infectious Diseases - PMC - NIH [3] Overview of PCR-based Diagnostic Assays for Emerging ...
SEE THE IDEA
PCR workflow
Extract and purify nucleic acid from the biological sample.
Short primers match the target sequence and mark what will be copied.
Denaturation, annealing, and extension repeat many times.
Product is seen by gel, fluorescence, or melting behavior.
Sequential process: preparation sets up targeted copying, cycling makes many copies, and detection shows whether the target was amplified.
PCR turns a small selected nucleic acid target into a detectable product by repeated temperature-controlled copying.
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.