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--- name: ? status: compiling version: 0.0.0 maintainer: Neo dependencies: [patience] ---
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--- name: Polymerase Chain Reaction (PCR) type: biological_process status: running version: 3.1.0 released: 1983-12-16 maintainer: Kary Mullis (archived); currently maintained by every molecular biology lab on Earth dependencies: - DNA template - Taq polymerase - dNTPs - primers (x2) - thermal cycler - someone who labeled the tubes correctly license: Nobel Prize (Chemistry, 1993); effectively public domain tags: - biology - diagnostics - forensics - molecular-cloning - pandemic-infrastructure - workhorse ---
A method for photocopying a specific segment of DNA so many times that you can actually work with it.
PCR runs in three repeating steps, each requiring a different temperature. This is not a metaphor. The machine literally heats and cools a tiny tube in precise cycles until the target sequence becomes undeniable.
| Step | Temperature | What happens |
|---|---|---|
| Denaturation | ~95°C | Double helix is forced apart. hydrogen bonds do not survive |
| Annealing | ~55-65°C | Primers bind to their complementary sequences |
| Extension | ~72°C | Taq polymerase synthesizes new DNA from each primer |
Repeat 30 to 40 times. Each cycle doubles the target. By cycle 30 you have over a billion copies of a sequence that started as a few molecules in a cheek swab.
This is exponential growth but useful.
ERR_NO_BAND // Product not amplified. Check template, primers, Mg2+ concentration, your life choices
ERR_SMEAR // Too many products. Raise annealing temperature
ERR_FALSE_POS // Contamination. Decontaminate bench. Decontaminate self
ERR_PRIMER_DIMER // Primers amplified each other. Redesign
ERR_THERMAL_DRIFT // Cycler is old. Replace cycler
Can PCR tell if someone is alive? No. It detects DNA. death does not immediately degrade it.
Is PCR ever wrong? Yes. See: ERR_FALSE_POS. A positive result means the sequence is present. It cannot tell you context, quantity, or what it means for the patient waiting outside.
Why does the lab smell like that? ethidium bromide. Do not ask further questions.