Figure 7.14.

Polymerase chain reaction (PCR).

PCR amplifies a specific sequence of DNA, based upon the location of primers. (A) In denaturation, high heat separates the two strands of the DNA sample. In annealing, the temperature is lowered, which allows the primers to hybridize to the strands. In extension, DNA polymerase extends the primers, creating the new strand. One DNA strand has now been copied into two. The three stages are repeated 30–35 times to generate billions of copies of the target sequence. If amplicons are created, then it means viral nucleic acid was present in the patient’s DNA sample. (B) A thermocycler, the machine that controls the reaction temperature during PCR. (The white machine on the right is another brand of thermocycler.)