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During the annealing phase in Polymerase Chain Reaction (PCR), the temperature is lowered to facilitate the binding of primers to the single-stranded DNA templates. This step is critical because primers are short sequences of nucleotides that provide a starting point for DNA synthesis. By decreasing the temperature, the conditions become suitable for the primers to attach or anneal to their complementary sequences on the target DNA. This precise alignment sets the stage for the next phase of PCR, where DNA polymerase extends the primers to synthesize new strands of DNA.

The other phases of PCR involve different processes. For example, increasing the temperature is necessary for denaturing the DNA, separating it into single strands. The activation of DNA polymerase occurs before the extension phase, where it catalyzes the synthesis of new DNA strands along the annealed primers. Understanding the annealing phase is essential for effective practical applications of PCR, such as cloning, sequencing, or diagnosing genetic disorders.