PCR: The Molecular Photocopier
Thirty cycles of heating and cooling turn one DNA molecule into a billion. It is the trick behind every DNA test — and it is just Unit 2's replication, run in a tube.
Builds on: 2.2 Replication: Copying Three Billion Letters3.1 Capstone: Extract DNA from a Strawberry
The problem: too little to see
A crime-scene hair, a drop of ancient bone, a nose swab early in an infection — each might hold a few hundred copies of the DNA you care about. No instrument on Earth detects that directly. The strawberry lesson needed millions of cells’ worth of DNA just to make threads you could see. What forensics, medicine and archaeology needed was an amplifier.
The solution: borrow replication
You already know a machine that copies DNA — Unit 2’s replication. The polymerase chain reaction puts a stripped-down version in a tube and drives it with nothing but temperature, cycling through three steps:
- 95 °C — denature. Heat breaks the hydrogen bonds and the double helix falls into two single strands. (GC-rich DNA, with its three bonds per pair, holds on hardest — Unit 2 again.)
- 55 °C — anneal. Two short synthetic DNA pieces called primers base-pair onto the strands, one on each side of the target region. Primers are the address label: only the stretch between them gets copied.
- 72 °C — extend. A polymerase builds new complementary strands starting from the primers. Two molecules now exist where one did.
Repeat. Each cycle takes about ninety seconds and doubles the count:
The enzyme from a hot spring
One catch: 95 °C destroys ordinary enzymes — that is what denaturation meant in Unit 0.3, and early PCR needed fresh polymerase added by hand every single cycle. The fix came from an organism that thinks 70 °C is comfortable: Thermus aquaticus, a bacterium from Yellowstone’s hot springs, whose polymerase (“Taq”) shrugs off the heat. Drop it in once and the whole reaction runs unattended in a benchtop thermal cycler.
Seeing the result
A billion copies is still invisible until you sort them. Gel electrophoresis uses a fact from the strawberry extraction: the phosphate backbone makes DNA negatively charged. Pull it through a gel with an electric field and short fragments slip through faster than long ones, so each length gathers into a band. Right length, right band, bright enough to photograph — that is a positive test.
Kary Mullis said the idea hit him on a California highway in 1983, and he pulled over to scribble the arithmetic. The chemistry was all known; the loop was the invention. It won the 1993 Nobel Prize and sits under every COVID PCR test, paternity test and genome project since.
⚗️ Lab — The Thermal Cycler
Three temperatures on repeat, and an exponential doing the rest.
- Step the cycles up one at a time and watch the copy count — each bar is ×32 the one before.
- Find how many cycles one molecule needs to become detectable (~10⁶ copies).
- Drop the efficiency to 85%. Compounding works against you just as hard.
Work it by hand — 0 / 2
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±2% count.