Stage 1
The reactants are mixed together in a PCR vial. The mixture
contains the DNA which is to be amplified, the enzyme DNA polymerase,
small primer sequences of DNA and a good supply of the four
nucleotide bases A,T,C and G. The vial is placed in a PCR machine. |
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Stage 2
The reaction mixture is heated to 90-95oC for about
thirty seconds. At this temperature the DNA strands separate
as the hydrogen bonds holding them together break down. |
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Stage 3
The mixture is cooled down to 55-60oC. At this temperature
the primers bind (or anneal) to the single DNA strands.
The primers are short sequences of nucleotide bases which must
join to the beginning of the separated DNA strands for the full
copying process to start. |
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Stage 4
In the final step the mixture is heated up again to 75oC
for at least a minute. This is the optimum temperature for the
DNA polymerase enzyme. The enzyme adds bases to the primers
segments to build up complementary strands of DNA identical
to the original molecule. |
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These
last three steps can be repeated around thirty times to give around
1 billion copies of the original DNA. The whole process takes
only about 3 hours – and much of that is the time
taken heating and cooling the reaction mixture in the PCR machine
(left).