What does the polymerase chain reaction do?

• A. It sequences the entire genome
• B. It enables scientists to produce millions of copies of a specific DNA sequence from a small amount of DNA
• C. It fragments DNA for analysis
• D. It converts RNA into DNA

Answer: (B)

PCR focuses on making many copies of a specific DNA segment. By cycling through heating and cooling, the target DNA denatures, primers bind to the ends of the region you want to copy, and DNA polymerase extends new strands. Each cycle doubles the amount of the target, so a tiny starting sample yields millions of copies. This amplification is what allows scientists to detect, analyze, or sequence that particular DNA region even from a small amount of material, and it underpins diagnostics, forensics, and research.

The other ideas aren’t what PCR does: sequencing an entire genome involves reading DNA sequences rather than just multiplying a chosen region; fragmenting DNA is a separate preparatory step; converting RNA into DNA is done by reverse transcriptase in RT-PCR, not by standard PCR.