PCR (Polymerase Chain Reaction): It is the revolutionary method developed by Kary Mullis in the 1980s. PCR is based on using the ability of DNA polymerase to synthesise new strand of DNA complementary to the offered template strand. Because DNA polymerase can add a nucleotide only onto a pre-existing 3'-OH group, it needs a primer to which it can add the first nucleotide.
Polymerase used for PCR is extracted from:

Explanation

**Step-by-Step Solution:**

1. **Understanding PCR**: The Polymerase Chain Reaction (PCR) is a technique used to amplify DNA. It was developed by Kary Mullis in the 1980s and has become a crucial method in molecular biology.

2. **Role of DNA Polymerase**: The key enzyme in PCR is DNA polymerase, which synthesizes new strands of DNA complementary to the template strand. It can only add nucleotides to a pre-existing 3'-OH group, which is why a primer is necessary.

3. **Source of DNA Polymerase**: The DNA polymerase used in PCR is extracted from a bacterium known as *Thermus aquaticus*. This bacterium thrives in high-temperature environments, which is essential for the PCR process.

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