Final answer:
The step of PCR that involves using extra DNA base pairs not in the starting molecule is the extension phase (c), where DNA polymerase synthesizes new strands by adding nucleotides.
Step-by-step explanation:
The step of PCR that requires using extra DNA base pairs that aren't in the starting molecule is extension (c). During the extension step, the DNA polymerase adds complementary nucleotides to the annealed primer on each DNA strand, thereby synthesizing new DNA strands. The denaturing step (a) involves separating the two strands of DNA by heating, and annealing (b) involves the binding of primers to these single-stranded DNA sections. It is in the extension phase where the new DNA is actually synthesized.
The polymerase chain reaction (PCR) is a technique used to make multiple copies of a segment of DNA. This process typically involves three main steps: denaturing, annealing, and extension.