WEBVTT
Kind: captions
Language: en

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Polymerase chain reaction, or PCR, uses
repeated cycles of heating and cooling

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to make many copies of a specific region of DNA.

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First the temperature is raised to near
boiling, causing the double-stranded

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DNA to separate or denature into single strands.

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When the temperature is decreased,
short DNA sequences known as primers

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bind or anneal to complementary
matches on the target DNA sequence.

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The primers bracket the target sequence to be copied.

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At a slightly higher temperature, the
enzyme Taq polymerase, shown here in blue,

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binds to the prime sequences and adds
nucleotides to extend the second strand.

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This completes the first cycle.

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In subsequent cycles, the process of
denaturing, annealing, and extending are

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repeated to make additional DNA copies.

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After three cycles, the target sequence
defined by the primers begins to accumulate.

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After thirty cycles, as many as a billion
copies of the target sequence are

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produced from a single starting molecule.