What occurs during the annealing step of PCR?

• A. The temperature is raised to facilitate the binding of DNA strands
• B. The temperature is lowered to allow primer binding to complementary DNA sequences
• C. The DNA polymerase synthesizes new DNA strands
• D. The new DNA strands are separated from the original strands

Answer: (B)

During the annealing step of PCR (Polymerase Chain Reaction), the temperature is lowered to enable the binding of primers to their complementary sequences on the single-stranded DNA template. This step typically occurs after the denaturation phase, where the double-stranded DNA is heated to separate it into two single strands. Lowering the temperature allows the primers, which are short DNA sequences designed to match the target region of the DNA, to hybridize effectively to the complementary regions on the single-stranded DNA templates. This precise binding is critical for ensuring that the subsequent extension phase can produce new DNA strands that are accurately complementary to the original template.

The correct understanding of this step emphasizes the importance of temperature in facilitating the specific hybridization of the primers, which is foundational for the success of the PCR process in amplifying target DNA sequences.