Explain PCR and its key steps (denaturation, annealing, extension) and its purpose in studying DNA replication.

• A. PCR amplifies a targeted DNA sequence through cycles of denaturation (separating strands), annealing (primer binding), and extension (DNA synthesis by polymerase).
• B. PCR uses RNA as a template to produce complementary DNA.
• C. PCR synthesizes RNA from DNA.
• D. PCR ligates DNA fragments together.

Answer: (A)

PCR is a method that amplifies a targeted DNA sequence by cycling through denaturation, annealing, and extension using a DNA polymerase. In denaturation, the double-stranded DNA is heated to separate into single strands. In annealing, primers bind to complementary sequences on each strand. In extension, the polymerase extends from the primers, synthesizing new DNA in the 5' to 3' direction. Repeating these steps exponentially increases the target DNA, producing millions of copies from a tiny starting amount.

This amplification is useful for studying DNA replication because it provides enough DNA to analyze a specific region, confirm its sequence, detect mutations that could affect replication, or measure copy number related to replication activity. It enables downstream analyses like sequencing or mutation detection to investigate replication dynamics and fidelity.

Other options describe processes not part of PCR: using RNA as a template is a variant called RT-PCR; synthesizing RNA from DNA is transcription; ligating DNA fragments is a separate technique.