What happens during the denaturing phase in a PCR cycle?

During the denaturing phase in a PCR (Polymerase Chain Reaction) cycle, the two strands of the double helix separate. This step typically occurs at elevated temperatures, usually between 90-98°C, which effectively breaks the hydrogen bonds between the complementary base pairs, resulting in the separation of the double-stranded DNA into single strands. This separation is crucial for the PCR process because it allows the DNA polymerase and primers to access the single-stranded template during the subsequent annealing phase. Without this denaturation step, the DNA would remain in its double-helical form, preventing the amplification of the target sequence.

The other phases of PCR—replication, amplification, and primer addition—do not occur during this specific phase, making the separation of the DNA strands the defining characteristic of the denaturing step.