What is the purpose of a primer in PCR?

• A. To denature the DNA
• B. To provide a starting point for DNA synthesis
• C. To ligate fragments
• D. To measure concentration

Answer: (B)

Primers in PCR provide the starting point for DNA synthesis. They are short, single-stranded DNA sequences that bind to complementary regions flanking the target sequence during the annealing step. DNA polymerase can only add nucleotides to a pre-existing 3' end, so the primers give the enzyme a free 3' hydroxyl group to extend from, enabling amplification of the specific region between the primers. This is why two primers are used to define the boundaries of the segment to be copied. Denaturing the DNA is a separate step that separates strands, ligating fragments is not part of PCR, and measuring concentration isn’t accomplished by primer function.