# Polymerase chain reaction

PCR copies a chosen stretch of DNA millions of times. Repeated cycles of heating (to separate strands), cooling (so primers bind) and warming (so heat-stable polymerase copies the DNA) double the amount each time.

Biology (university) · Part 05: Gene regulation and biotechnology · Card 6 of 8 · Last updated 2026-10-19

## Analogy

A photocopier that doubles the pile with every press.

## Example

**Doubling**

After 10 cycles: 2^10 = 1,024 copies.
After 30 cycles: about
1 billion copies.

## In practice

PCR can amplify the tiny amount of DNA in a drop of blood so it can be tested.

## Practice

### Flashcards

- **Why does PCR use a heat-stable polymerase?** It must survive the repeated heating that separates the DNA strands.

### Interview questions

**Explain the three steps of a PCR cycle and why the technique is so useful.**

- Denaturation separates strands
- Annealing lets primers bind
- Extension by heat-stable polymerase
- Number of copies doubles each cycle
- Makes tiny samples testable

## Questions people ask

### Who invented PCR?

Kary Mullis developed PCR in the 1980s and shared the 1993 Nobel Prize in Chemistry for it. Its use became practical once a heat-stable polymerase from the hot-spring bacterium Thermus aquaticus, called Taq, was introduced. It saved researchers from having to add fresh enzyme after every heating step.

### What are primers in PCR?

Primers are short pieces of single-stranded DNA, usually 18 to 25 bases long, that match the two ends of the region to be copied. DNA polymerase needs them to start building. Because they only bind to the matching sequence, they make sure that only the chosen region is copied.

## Related concepts

- [Gel electrophoresis](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-7)
- [Restriction enzyme](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-4)
- [Recombinant DNA](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-5)
- [CRISPR-Cas9](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-8)

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