Suggest why two different primers are required in PCR.

• A. Because the sequences at the ends of the target sequence are identical
• B. Because one primer is enough to initiate synthesis
• C. Because the ends of the target sequence differ and define the region to amplify
• D. Because primers compete with nucleotides

Answer: (C)

PCR needs two primers because it amplifies a specific DNA fragment bounded by those primer binding sites. Each primer attaches to opposite ends of the target region on opposite strands and is oriented to face toward each other, so DNA polymerase can extend from both primers and copy the DNA in between. Since the ends of the target sequence are different, two distinct primer sequences are required to define the boundaries of the amplified fragment. Using only one primer wouldn’t specify a unique region to copy, leading to non-specific or no defined product.