Why are DNA primers added during PCR?

• A. To mark the beginning and end of the target region
• B. To attach nucleotides to the growing chain
• C. To provide a starting point for DNA synthesis by defining the region to be amplified
• D. To denature the DNA strands

Answer: (C)

Primers provide the starting point for DNA synthesis by defining the region to be amplified. They are short DNA sequences that are complementary to the ends of the target segment, so they bind to the template and create a free 3' hydroxyl group for the DNA polymerase to extend from. In PCR, two primers flank the region of interest and determine exactly which segment gets copied; after each cycle, these same primers bind to the newly formed strands, enabling exponential amplification of that specific region. While the polymerase adds nucleotides, it needs a primer to begin, and the primers establish the boundaries of the amplified fragment. Heat or other steps denature DNA, and polymerase action—not primers—adds the nucleotides.