PCR and eDNA: a plain-English guide to how amplification works

Every eDNA test relies on polymerase chain reaction — PCR — to turn a handful of DNA molecules into billions of copies a sequencer can read. PCR is the reason you can detect a fish from a liter of water. It is also the source of most eDNA false positives and false negatives.

What PCR does

PCR is molecular photocopying. Short synthetic sequences called primers grab onto specific target DNA, and a heat-stable enzyme copies the region between them. Cycling through temperature changes doubles the copies each round, so after 30 to 40 cycles you have gone from a few molecules to billions.

This exponential amplification is what makes eDNA possible. Environmental samples contain vanishingly small amounts of DNA from any one species, and no sequencer can read a handful of molecules directly.

Why primer choice determines what you see

PCR only copies what the primers bind to. If you use vertebrate primers, insect DNA is invisible. If you use fungal primers, plants are invisible. This is a feature — it lets you focus a test on the target group — but it means one test cannot see everything.

Universal primers exist, but they always have blind spots and biases. A comprehensive survey uses several primer sets in parallel.

How PCR can lie

Contamination is the biggest risk. PCR is so sensitive that a single stray DNA molecule from lab air, a reagent, or the technician can be amplified into a false positive. Good labs run negative controls at every step and are ruthless about flagging suspicious hits.

Inhibition is the opposite failure. Compounds in soil and water can partially block the polymerase, suppressing signal from species that were genuinely present — a false negative. Careful extraction and inhibitor-tolerant enzymes minimize this.

qPCR vs metabarcoding

Quantitative PCR (qPCR) uses primers specific to one target species and measures how much DNA is present. It is fast, cheap, and very sensitive — the standard tool for surveillance of one invasive or endangered species.

Metabarcoding uses broader primers, sequences everything they amplify, and reports the whole community. It is the standard tool for inventory work. Most consumer eDNA kits are metabarcoding-based.

Frequently asked questions

Can PCR detect one molecule?

In theory yes, in practice detection limits are usually a handful of target molecules per reaction. That is still extraordinary sensitivity by any biological standard.

Why do reputable labs run negative controls?

Because PCR is sensitive enough that lab contamination will otherwise show up as false positives. Negatives are how a lab proves a hit is real.