This animation explains the three key steps involved in the PCR process. The three steps form a cycle that is repeated many times to produce millions of copies of a specific DNA sequence from a small initial sample.
The polymerase chain reaction or PCR is used to make multiple copies of a specific sequence of DNA called the target DNA.
The DNA is collected from a sample of interest – for example, someone’s hair follicle, material from a crime scene, an ancient bone, a plant seed or cancerous tissue.
The process requires a special machine called the PCR machine or thermocycler.
A solution containing the target DNA is placed in the machine. This solution contains the target DNA sample, primers, nucleotides, the enzyme DNA polymerase, buffer and magnesium chloride.
PCR involves the repetition of a series of three steps.
In the first step, the temperature is increased to above 90°C. This heating breaks the hydrogen bonds between the complementary base pairs, causing the two strands in the DNA double helix to come apart or denature.
In the second step, the temperature of the solution is lowered. This causes the primers to attach or anneal to the DNA. Primers match specific sequences of DNA. They are needed to start the process of making new DNA.
The third step requires the enzyme DNA polymerase. This enzyme is used to form new DNA strands. The temperature is again increased, and the polymerase helps make new DNA strands that match the DNA strands that were separated.
The three-step cycle is then repeated many times.
The temperature is increased and the DNA strands separate. The primers anneal and the polymerase helps make new DNA strands. Then it starts again. The DNA strands are separated. The primers anneal, new DNA strands are made and so on.
With every cycle, the number of copies of the target DNA doubles. After only 20 cycles, you have over a million copies of the original DNA.
Root of hair follicle, Wellcome Images, Creative Commons 4.0
Crime scene image, Paul Fleet, licensed through 123RF Limited
Drilling collagen from bone, Frank Vinken, Max Planck Institute for Evolutionary Anthropology
Kōwhai seeds, Roger Culos, Muséum de Toulouse, Creative Commons 3.0