PCR VCE Biology 3/4

unit 3 AOS 1

PCR

A technique used to amplify a specific piece of DNA, making many identical copies.

What is the purpose of PCR?

To produce many copies of DNA when only a small or trace sample is available.

Name four uses of PCR.

Crime-scene DNA, fossil DNA, pathogen testing, and paternity testing.

What is the starting DNA sample in PCR?

The DNA containing the target region to be copied.

What are DNA primers?

Short single-stranded DNA sequences, about 10–20 bases long, complementary to regions flanking the target.

Why are forward and reverse primers used?

They bind to opposite strands at either end of the target region and provide starting points for copying.

What is the role of free nucleotides in PCR?

They are the A, T, G and C building blocks used to synthesise new DNA strands.

What is the role of DNA polymerase in PCR?

It builds new DNA strands by adding complementary nucleotides.

What is Taq polymerase?

A heat-resistant DNA polymerase extracted from the bacterium Thermus aquaticus.

Why is Taq polymerase used in PCR?

It withstands the high temperatures used to separate DNA strands.

What happens during denaturation in PCR?

The mixture is heated to about 95°C, breaking hydrogen bonds and separating DNA into single strands.

What happens during annealing in PCR?

The mixture is cooled to about 55°C so primers bind to complementary target sequences.

What happens during elongation in PCR?

At about 72°C, Taq polymerase extends from the primers by adding complementary nucleotides.

Why are PCR stages repeated?

Repeated cycles amplify the target DNA, producing millions of copies.

How does the number of DNA copies change each PCR cycle?

Ideally it doubles each cycle, following 2ⁿ after n cycles from one starting target.

How many target DNA copies are present after 10 ideal PCR cycles?

1,024 copies.

How does PCR relate to DNA replication?

PCR mimics aspects of DNA replication but uses controlled temperature changes in a tube.