Real crime-scene samples are rarely generous. A single drop of dried blood, a licked envelope, a few cells on a cigarette end — often that's all there is. So before a laboratory can read a DNA profile, it has to make more DNA. The technique that does this, the polymerase chain reaction (PCR), can turn the DNA from a few cells into billions of copies of the exact regions the analyst wants to read.

What's in the tube

PCR is a controlled chemical copying reaction. To run it you need:

  • Template DNA — the sample. In a bloodstain, the DNA comes from the white blood cells (red cells have no nucleus).
  • Primers — short synthetic DNA sequences that flank the target region and tell the reaction where to start copying. Choose primers around an STR locus and you copy that locus.
  • Taq polymerase — a heat-stable enzyme (originally from the hot-spring bacterium Thermus aquaticus) that builds the new DNA strands. Its heat tolerance is what makes the cycling possible.
  • Nucleotides (dNTPs) — the A, T, C and G building blocks.
  • Buffer and magnesium, all held in a thermal cycler that heats and cools the tube precisely.

One cycle, three temperatures

Each PCR cycle is just three temperature steps:

  1. Denaturation (~94–95 °C). The heat breaks the hydrogen bonds holding the double helix together, so the two strands separate into single strands.
  2. Annealing (~55–60 °C). The tube cools and the primers bind to their matching sequences on each single strand, bracketing the target region.
  3. Extension (~72 °C). Taq polymerase starts at each primer and adds nucleotides, building a new complementary strand — so each single strand becomes a double strand again.

At the end of one cycle, every target molecule has become two.

Why it explodes: exponential copying

That doubling is the whole trick. Repeat the three steps and the copies double each cycle: 1 → 2 → 4 → 8 → 16… After roughly 28–32 cycles, a handful of starting molecules becomes on the order of a billion copies of the target — more than enough to detect and measure. From a single drop of blood, the lab now has abundant material to work with.

Copying many markers at once

Modern forensic kits don't amplify one region at a time. They run a multiplex PCR, copying all the STR loci of a system such as SGM+ in a single reaction, using primers tagged with fluorescent dyes. That colour-coding is what lets the next instrument tell the loci apart.

A double-edged sensitivity

The same power that lets PCR work from almost nothing is also its danger: it will amplify contaminant DNA just as eagerly as the sample. This is exactly why scene and laboratory anti-contamination discipline matters so much — a few stray cells, multiplied a billion times, become a real signal.

What happens next

Amplification doesn't produce a profile by itself. The billions of copies are fed into capillary electrophoresis, which sorts the fragments by size and reads the fluorescent tags — producing the peaks that the analyst translates into the numbers of a DNA profile.

So the journey from a drop of blood to a courtroom profile begins here: three temperatures, repeated thirty times, quietly turning a trace into something a scientist can actually read.