Final answer:
Primers are specific sequences of DNA that bind to complementary sites on the DNA strands to allow DNA polymerase to begin the replication process in PCR. They are designed to flank the gene that needs to be amplified, ensuring the specificity of the amplification. Therefore, the correct option is D.
Step-by-step explanation:
The two primers bind to specific sites in the DNA and flank the gene to be amplified. Primers are short stretches of DNA that are synthesized to be complementary to the 3'-ends of the strands of DNA that contain the gene of interest. By attaching to these specific sites, they enable the DNA polymerase to initiate the replication process, which is essential for PCR (Polymerase Chain Reaction), a technique widely used in molecular biology to amplify a specific DNA segment.
A primer is needed because DNA polymerases can only add nucleotides to an existing strand of DNA. It acts as a starting point for DNA synthesis. The specificity of the PCR process depends largely on the sequence of the primers, as they define the beginning and end of the region of the DNA that will be amplified.