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Molecular Biology · Process

PCR Diagram

How PCR copies one stretch of DNA: heat the strands apart, cool so primers bind, let Taq build new strands, and repeat until the copies number billions.

The polymerase chain reaction makes millions to billions of copies of one chosen stretch of DNA from a tiny starting amount. A thermal cycler takes the tube through the same three temperatures again and again, and each cycle doubles the copies. Step through one cycle, then what repeating it does.

Also called Polymerase chain reaction.

PCR. Across the top, the temperature of one cycle: 95 degrees Celsius to separate the strands, 50 to 65 degrees for primers to bind, 72 degrees for Taq polymerase to extend them, then repeat 25 to 35 times. Below, four panels: the two template strands separating; a primer binding at each end of the target, one on each strand; Taq polymerase building new strands from the primers out of free dNTPs; and a bar chart of copies doubling each cycle, 2, 4, 8, 16 and 32.95 °C · 20 to 30 s50 to 65 °C · 20 to 40 s72 °C · 1 min per kbRepeat 25 to 35 times1 Denaturation2 Annealing3 Extension4 Many cyclesTargetStrands separatePrimers bindPrimers pair with the endsTaq polymeraseNew strands growFree dNTPs122438416532Cycle: copies double
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Step 1 of 7

PCR at a glance

PCR copies one stretch of DNA again and again. Each cycle has three temperatures: hot to separate the strands, cooler for primers to bind, and warm for the polymerase to build new strands. Every cycle doubles the copies.

Use ← and → to move between steps.

Step 1 of 7: PCR at a glance

Download the diagramChecked against its sources on 25 October 2026.

Step by step

The steps of the PCR

  1. 01PCR at a glance

    PCR copies one stretch of DNA again and again. Each cycle has three temperatures: hot to separate the strands, cooler for primers to bind, and warm for the polymerase to build new strands. Every cycle doubles the copies.

  2. 02What goes in the tube

    The tube holds the template DNA, two primers, a heat-stable DNA polymerase such as Taq, the four dNTPs, and a buffer with magnesium ions, which the polymerase needs. A thermal cycler then heats and cools it.

  3. 03Denaturation at about 95 °C

    Heating to 94 to 98 °C for 20 to 30 seconds breaks the hydrogen bonds between the two strands, and the double helix separates into single strands. The first cycle often starts with a longer heat, of a few minutes.

  4. 04Annealing at about 55 °C

    Cooling to about 50 to 65 °C, usually a few degrees below the primers' melting temperature, lets the primers bind to their matching sequences, one at each end of the target and one on each strand.

  5. 05Extension at 72 °C

    At 72 °C, close to the best temperature for Taq, the polymerase adds dNTPs to the 3′ end of each primer, building a new strand that matches the template. About a minute is allowed for every 1,000 bases.

  6. 06Each cycle doubles the copies

    Every new strand is a template in the next cycle, so the copies double each time: up to 2ⁿ after n cycles, about a billion after 30. Copies exactly the length of the target first appear in the third cycle and soon make up nearly all the product.

  7. 07The end of the run

    Late in the run the reaction slows to a plateau: reagents run low, product strands pair with each other, not with primers, and the polymerase loses activity. A final few minutes at 72 °C finish any part-made strands, and the product is usually checked by gel electrophoresis as a band of the expected size.

Parts

What each part of the pcr does

Template DNA
The DNA that holds the stretch to be copied. A few nanograms can be enough, since every cycle doubles what there is.
Target sequence
The stretch of DNA between the two primers, often a few hundred to a few thousand base pairs long. It is what the reaction copies.
Primers
Two short single strands of DNA, usually 18 to 25 nucleotides, made to match the two ends of the target: one for each strand. They decide what gets copied.
Taq polymerase
A DNA polymerase from the hot-spring bacterium Thermus aquaticus. It survives the 95 °C steps and works best at about 70 to 80 °C.
dNTPs
The four deoxynucleotide triphosphates (dATP, dCTP, dGTP and dTTP), the building blocks the polymerase adds to the growing strands.
New strands
Built from each primer's 3′ end, complementary to the template. In later cycles they become templates themselves.
Denaturation
At 94 to 98 °C the hydrogen bonds between the strands break, and the double helix separates into two single strands.
Annealing
At about 50 to 65 °C, a few degrees below the primers' melting temperature, the primers bind to their matching sequences.
Extension
At 72 °C Taq polymerase extends each primer; protocols allow a minute per 1,000 bases.
Thermal cycle
One round of the three temperatures, a few minutes long. A thermal cycler repeats it 25 to 35 times.
Exponential copying
Each cycle doubles the copies of the target, so n cycles give up to 2ⁿ copies of each template molecule: 30 cycles, about a billion.

Dictionary

The terms to know

Practise it

The quizzes and cards that fit

In 3D

The molecules, to turn and explore

Next to it

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Sources

Where the facts come from

Drawn and written by biofideX from these sources.

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