Final answer:
Heat-stable DNA polymerases like Taq polymerase from thermophilic bacteria are essential for PCR because they can withstand repeated high-temperature cycles without denaturing, allowing DNA amplification without adding new enzyme after each cycle.
Step-by-step explanation:
Heat-stable DNA polymerases from thermophilic bacteria such as Taq polymerase from Thermus aquaticus are required for the polymerase chain reaction (PCR) because of their ability to withstand the high temperatures used during the reaction. The PCR process consists of multiple cycles, each including a high-temperature denaturation phase (around 95°C) to separate the DNA strands. Most DNA polymerases from eukaryotic or non-thermophilic bacteria would denature at this temperature, similar to how proteins in an egg coagulate when boiled. However, enzymes like Taq polymerase, which has evolved in high-temperature environments, remain active over a wide temperature range and are not denatured during the PCR cycles.
These heat-stable polymerases allow for the continuous amplification of DNA without the need to add fresh enzyme after each cycle. This makes the process more efficient and less costly since it eliminates the need for additional polymerase with each cycle. The introduction of these polymerases and the development of automated thermal cyclers have made PCR a widely used tool in molecular biology.