Polymerase Chain Reaction (PCR)

Artificial DNA replication

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DNA sample is mixed with DNA nucleotides and DNA polymerase enzyme

Heated to 95 degrees celcius to break H. bonds = becomes single stranded

Primers added (as DNA polymerase cannot bond directly to single stranded DNA)

Cooled to 55 degrees celcius

Primers bind (H. bonds) = now double stranded DNA at both ends

DNA polymerase binds to double stranded sections

Heated to 72 degrees celcius (optimum temperature for DNA polymerase)

Adds free nucleotides until reaches other end

=New double stranded DNA molecule formed

Repeated many times so amount of DNA increases exponentially

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PCR relies on that DNA

Antiparallel backbone strands

5' prime and 3' prime ends

Only grows from 3' end

Base pairing complementary rules

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Unlike natural DNA replication

Only able to replicate shorter DNA sequences (few hundred bases long)

Primer molecules required to start polymerase chain reaction

Heating and cooling cycle to separate and bind strands (H. bonds)


Entire chromosomes replicated

Primer molecules not required

DNA helicase separates strands (H. bonds)

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DNA polymerase:

Thermophillic so not denatured by the high temperatures used in the polymerase chain reaction

Taken from the thermophillic bacterium Thermus aquaticus (Taq), which grows in hot springs at 90 degrees celcius


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