Final answer:
The machine used to perform many cycles of PCR and change temperatures at exact times is known as a thermocycler. It facilitates the critical temperature fluctuations essential for the denaturation, annealing, and extension steps of the PCR process.
Step-by-step explanation:
To perform many cycles of Polymerase Chain Reaction (PCR), the machine that is required to change temperatures at exact times is called a thermocycler. During a PCR cycle, there are three temperature-dependent steps: denaturation, annealing, and extension. The thermocycler is programmed to cycle through these temperature changes, which include a high temperature for the denaturation of the DNA strands, a lower one for primer annealing, and an intermediate one for the extension of the new DNA by a DNA polymerase like Taq polymerase that is resilient to high temperatures. Scientists developed thermal cyclers capable of quickly and precisely modulating temperatures, and this innovation alongside heat-stable DNA polymerases, allowed PCR to become widely accessible in the 1980s.