Answer:
Because of the several steps with high temperatures and also (heavy ) changes in temperature.
Explanation:
PCR consists of a serie of 20–40 repeated temperature changes. Those series or cycles are also called thermal cycles.
The individual steps (common to most PCR methods) are as following:
→Denature template : 99 °C for 2 minutes
→Anneal primers to template : 55 °C for 2 minutes
→Extension of the primers by heat stable DNA polymerase :72 °C for 2 minutes
As we can notice, will in a relatively short time, different temperatures be used. It's important that the used polymerase will be functional at corresponding temperatures, as well as (heavy) temperature changes.
→ Since the bacteria Thermus aquaticus lives near thermal vents in the ocean floor and grows at temperatures of up to 98°C, it's used to corresponding temperatures.
If other enzymes will be used, i.e enzymes functional in lower temperatures (several) steps will not continue or will not be executed properly. To avoid this it's important to use the best temperature resistant enzymes.
