In PCR (Polymerase Chain Reaction), the denaturation step (~94°C) breaks hydrogen bonds between the complementary strands of double-stranded DNA, producing single-stranded templates. This is essential because DNA polymerase can only synthesise a new strand on a single-stranded template. The three steps — denaturation → annealing → extension — are repeated ~25–35 cycles, exponentially amplifying the target sequence.