The polymerase chain reaction (PCR) devised by Kary Mullis is an in vitro method for producing large amounts of essentially identical copies of a specified DNA sequence. This technique has revolutionized molecular genetics.
PCR exploits certain features of natural DNA replication. DNA polymerase uses single-stranded DNA as a template for the synthesis of a new complementary strand. These single-stranded DNA templates can be produced by heating double-stranded DNA to temperatures near boiling. The initiation of the DNA synthesis requires a small section of double-stranded DNA known as primers. Figure 1 shows the example used in [5]. Hence we can specify the starting point for DNA synthesis by supplying an oligonucleotide primer that anneals to the template at that point. This is the first prominent feature of PCR, that is, we can specify amplification regions by choosing proper primers.
If we supply a primer for each of the two strands, then they can
both be used as
templates for synthesis. For a PCR, the primers are chosen to flank
the region of DNA that is to be amplified. The newly
synthesized strands of DNA, starting at each primer, extend beyond the
position of the primer on the opposite strand, so that they both
contain new primer
binding sites for further replication.
Then this reaction mixture is
heated again to denature both the original and new double-stranded
sequences,
and form templates for the next cycle of primer hybridization, DNA
synthesis, and strand separation.
Each cycle consists of three steps: strand separation or DNA
denaturing, primer annealing, and DNA synthesis or polymer extension.
After n cycles, ideally, the mixture will have 
double-stranded
DNA molecules that identical to the original sequences between the
two primers. This is the second prominent feature of
PCR---amplification.
Figure 1: Primers for DNA polymerase. (a) The target for amplification,
a small section covering 110 bp of the 
--globin, is
shown. Two sequences separated by 60 nucleotides are detailed, and
the 20 nucleotides underlined are used as oligonucleotide primers
for the PCR. (b) When the DNA is heated, the strands separate. (c)
The oligonucleotide primers (shown in green) hybridize specifically
to their complementary sequences at the 3' ends of each strand of
the target sequence. (d) DNA polymerase uses these primers to begin
synthesis of new strands (shown in orange and underlined)
complementary to the target DNA sequences in the 5'--to--3'
directions.
The specificity of PCR is determined by the oligonucleotide primers. This feature allows DNA from a variety of sources to be amplified by PCR. PCR does not require highly purified DNA, and enables the amplification from a very tiny number of DNA molecules.