Which step in the PCR cycle breaks hydrogen bonds and separates the DNA strands?

• A. Ligation step.
• B. Annealing step.
• C. Denaturation step breaks hydrogen bonds and separates the strands.
• D. Extension step.

Answer: (C)

The step that breaks hydrogen bonds and separates the DNA strands is the denaturation step. In PCR, you heat the reaction to a high temperature (roughly 94–98°C), which provides enough energy to overcome the hydrogen bonds holding the base pairs together. As those bonds break, the double-stranded DNA melts into single strands, giving templates for the next step. After denaturation, primers can anneal to their complementary sequences in the following step, and DNA polymerase then extends the new strands. Ligation isn’t part of the PCR cycle; it’s used in other cloning applications to join DNA fragments.