PCR: copying a chosen DNA segment — Biology, 14–17

How a laboratory makes millions of copies of a small DNA region so it can be detected and studied.

A molecular photocopier

PCR repeatedly copies one chosen stretch of DNA. Short primers mark its two ends, and a heat-stable enzyme builds matching strands, so a tiny starting sample becomes easy to detect.

Why copy DNA?

A few DNA molecules are too little to see or measure confidently. PCR solves this sampling problem by amplifying a selected region, rather than copying every part of the genome equally.

Three PCR cycles

Start with 1 double-stranded DNA molecule containing the target. After copying, cycle 1 gives 2 molecules; cycle 2 gives 4; cycle 3 gives 8. Ideally, each cycle doubles the target DNA.

PCR does not create a whole organism

Because PCR makes millions of copies, it can sound like cloning a living thing. That is a reasonable association, but PCR copies a short DNA sequence in a tube; it does not build cells or an organism.

A tool with many jobs

PCR helps identify pathogens, match crime-scene samples, test inherited variants and detect tiny traces in food or water. The result depends on careful sampling and controls, not just on the machine.

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