What is PCR? Describe the steps in one cycle of PCR.

1. 1 · Define in one line. PCR, the polymerase chain reaction, makes many copies of a chosen DNA segment in vitro using primers and a DNA polymerase.
2. 2 · Denaturation. High temperature separates the double-stranded DNA into single strands.
3. 3 · Annealing. On cooling, two primers bind to complementary sequences at the ends of the target.
4. 4 · Extension. Taq polymerase, thermostable and from Thermus aquaticus, adds nucleotides to the primers.
5. 5 · Draw the cycle. [Diagram: DNA, separated strands, primers bound, new strands growing, two copies]
6. 6 · Close in one line. Repeating the cycle about 30 times gives about a billion copies.

Scoring tip: Name all three steps in order and say why the polymerase must be thermostable.