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Purpose- to produce large numbers of copies of a gene, or amplify rare copies of a piece of DNA.
For example a Forensic scientist might want to amplify DNA in a hair found from a crime scene in
order to analyse it. DNA polymerase is added to the sample. It will catalyse millions of copies of
the small sample when treated appropriately, with the correct temperature and enzymes. Used as
the starting point for genetic sequencing.
Three steps, repeated for up to 40 cycles.
1. DNA strand heated to 94C to denature it and form single strands. This is where it differs
from the natural process as in the natural process DNA Helicase would do this rather than
heat. All enzyme actions stop.
2. Annealing at 54C. During this process the Primers undergo Brownian motion. Ionic bonds
are formed between the primer strands and the template.in areas where the bonds fit
more exactly, the bonds last longer, allowing polymerase to start copying the
template.(The Polymerase comes from a thermophillic bacterium found in hot springs)
3. Extension at 72C. This is the optimum temperature for the polymerase. The bases that are
complimentary to the template pair up from the 3' side.
4. Because both strands are copied during the process the rate of increase is exponential
(the rate of change increases with time).
PCR is used to test for conditions such as alterations in the CTFR gene, which may cause Cystic
Fibrosis, and alterations in the gene, which may cause some forms of Cancer. It can also be used
for paternity testing.


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