To read a single sentence you first need many copies. PCR is a molecular photocopier that doubles a snippet of DNA again and again until there's enough to sequence — to spell out its letters and read the genetic message.
Amplify, then read
The polymerase chain reaction (PCR) makes millions of copies of a target DNA segment. Repeated cycles of heating and cooling denature the double helix, let short primers anneal to the flanking sequence, and let a heat-stable polymerase extend new strands — doubling the DNA each cycle. Sequencing then determines the exact order of bases (A, T, C, G).
Each PCR cycle roughly doubles the target, so n cycles yield about 2ⁿ copies — 30 cycles turn one molecule into a billion, enough to detect and sequence.
PCR is the basis of many diagnostic tests: amplify a pathogen's DNA/RNA until it's detectable, even from a tiny sample.
- Recall that DNA's two strands must separate before copying.
- Note that primers must then bind, and polymerase must extend.
- Explain why each step needs a different temperature.
What you should see: High heat denatures (separates) the strands, cooling lets primers anneal, and a mid temperature lets the polymerase extend — so cycling the temperature drives each stage of copying.