Final answer:
In PCR, DNA strands separate during the denaturation stage, which involves heating the DNA to cause the two strands to separate, forming single strands.
Step-by-step explanation:
The stage at which the DNA strands separate during a Polymerase Chain Reaction (PCR) is known as denaturation. This step involves heating the DNA to a temperature typically around 95 ºC, which breaks the hydrogen bonds holding together the two DNA strands. This results in the formation of two single DNA strands, providing the necessary template for subsequent steps of the PCR process.