Final answer:
The energy for adding nucleotides during PCR comes from the hydrolysis of two phosphates from the incoming nucleoside triphosphate, which DNA polymerases use to catalyze the formation of phosphodiester bonds.
Step-by-step explanation:
During PCR, the energy for adding a nucleotide to the growing DNA strand comes from the nucleotide itself, which is a nucleoside triphosphate. DNA polymerases catalyze the formation of phosphodiester bonds, and this process is energetically driven by the hydrolysis of two phosphates (pyrophosphate) from the incoming nucleotide. The bond between these phosphates is high-energy, and when it is broken, the energy released is used to form the new bond between the incoming nucleotide and the growing DNA chain.