Polymerase Chain Reaction (PCR)

Primers are short nucleotide sequences that are made in the laboratory. They are recognized by DNA polymerases as the START tags for building complementary sequences of DNA dictated by computer programs stored in PCR machines.

Before PCR can be conducted, one must first determine the nucleotide sequences just before and after the gene to be copied. Then complementary primers are created.

To conduct the PCR reactions researchers mix primers, DNA polymerase, cellular DNA from an organism, and free nucleotides together.

Precise temperature cycles in the PCR machine cause the DNA strands to separate, exposing the bases. Primers become positioned on the exposed nucleotides to form new copies of the original DNA. Each round of reactions doubles the number of DNA molecules to eventually produce billions of molecules from very tiny amounts of original DNA.

DNA is heated to unwind strands and then cooled to allow base-pairing with primers and complementary strand synthesis. Then the DNA is heated again to unwind strands and the cycle is repeated over and over again.

Most DNA polymerase is denatured at high temperature. The polymerase used in PCR (called taq polymerase) is from bacteria that live in hot springs. The bacteria Thermus aquaticus was first isolated from hot springs in Yellowstone National Park.

Here are two animations that describe PCR:
PCR animation 1.
PCR animation 2.

REVIEW: PCR stands for
Answer polymerase chain reaction