What is PCR and what are its main steps?

• A. A method to sequence DNA using fluorescent labels.
• B. A technique to transcribe RNA into DNA.
• C. Polymerase chain reaction amplifies DNA; steps are denaturation, annealing, extension (repeated cycles).
• D. A process to separate proteins by size using gel filtration.

Answer: (C)

PCR is a method to amplify a specific DNA segment by cycling through three steps: denaturation, where the double-stranded DNA is heated to separate into two single strands; annealing, where primers bind to complementary sequences flanking the target region; and extension, where a DNA polymerase extends from the primers to make new DNA strands. Repeating these cycles many times doubles the amount of target DNA each cycle, giving exponential amplification. A thermostable polymerase is used so it can withstand the high temperatures during denaturation. This technique is different from sequencing methods that read DNA, reverse transcription that makes DNA from RNA, or protein separation techniques.