Polymerase chain reaction (PCR) is an in vitro DNA amplification protocol. It selectively amplifies a specific DNA sequence from any source (i.e. virus, bacteria, plant, human) hundreds of millions of times in a matter of hours. PCR is a technique necessary for several other genetic applications such as sequencing, RFLP analysis and microsatellite analysis.

Reaction Components
Template DNA contains the sequence of DNA you wanted to amplify. The DNA can be from animals, plants, viruses, or bacteria.

DNA Polymerase is an enzyme that makes a new strand of DNA through the sequential addition of nucleotides.

Primers are small segments of single-stranded DNA, usually 20 nucleotides long, which bind to a specific region on either side of the target DNA sequence and initiates replication of the target DNA at that point. Primers specify the DNA sequence to be amplified.

Buffer is a salt-solution that helps to stabilize the DNA and other components of the reaction.

PCR is repeated cycling of three steps:
  1. Denature DNA
    The DNA is heated to 95 C. This breaks the weak hydrogen bonds that hold DNA strands together in a helix, allowing the strands to separate creating single stranded DNA.
  2. Primer Annealing
    The mixture is cooled to anywhere from 45-72 C. This allows the primers to bind (anneal) to their complementary sequence in the template DNA.
  3. Extension
    The reaction is then heated to 72 C, the optimal temperature for DNA polymerase to act. DNA polymerase extends the primers, adding nucleotides onto the primer in a sequential manner, using the target DNA as a template.
  4. Go to Step 1
These steps are repeated 20-35 times. In PCR, amplification is exponential because for each cycle, the DNA made in the previous cycles can also serve as template (see figure below).

Multiple PCR cycles yield many copies of a DNA segment. After one cycle of PCR, one copy of DNA becomes 2, after 2 cycles it becomes 4, after 3 cycles you now have 8 copies and so on.
The figure illustrates the steps of PCR for the first two cycles. The reaction begins with just one copy of the target DNA. After one cycle of PCR there are two copies of the target DNA. Both of these copies serve as templates for the next cycle. At the end of the second cycle, there are now four copies of the target DNA. These steps would be repeated many more times, creating millions of copies of the target DNA.