Polymerase Chain Reaction

Polymerase Chain Reaction (PCR) is a laboratory technique used to rapidly make millions of copies of a specific DNA sequence in vitro (in a test tube). It involves cyclic heating and cooling of a DNA sample in the presence of DNA primers, nucleotides, and a heat-stable DNA polymerase enzyme, to exponentially amplify the target DNA segment.

• PCR steps: (1) Denaturation - heat the DNA so the double strand separates into single strands; (2) Annealing - cool the reaction so that primers (short DNA sequences) bind to the specific target sequence; (3) Extension - the heat-tolerant DNA polymerase (commonly Taq polymerase from bacteria *Thermus aquaticus*) synthesizes new DNA starting from the primers. These steps repeat for ~30 cycles, doubling the DNA each cycle, so a single copy can become billions.
• PCR is highly sensitive - it can amplify trace amounts of DNA. This makes it useful in DNA fingerprinting (forensics), detecting pathogens in patient samples, paternity testing, genetic research, etc. For example, PCR can be used to detect a virus by amplifying a segment of its genome from a patient's blood sample.
• A common exam question: understanding why a special DNA polymerase (Taq polymerase) is used in PCR - it's because PCR involves high temperatures (around 95°C for denaturation), and Taq polymerase is stable at high heat (won't denature). This enzyme can extend primers at ~72°C and withstand repeated heating cycles.

• If asked "which technique amplifies DNA sequences rapidly in vitro," the answer is PCR. Recognize descriptions of PCR: keywords include primers, Taq polymerase, thermal cycling, and DNA amplification.
• They may test understanding of PCR components: for example, a question might ask what is needed for PCR (template DNA, primers, DNA nucleotides, DNA polymerase, and thermal cycling conditions). Or ask why regular DNA polymerase won't work (because it would be inactivated by the high heat).
• Sometimes exams compare PCR to processes like DNA replication in cells or cloning vectors. Remember: PCR is performed in a lab tube (no cells needed) and is much faster, producing large amounts of DNA from a small sample by repeated cycles.