The enzyme used in Polymerase Chain Reaction is:

Question

Accepted Answer

The correct option is (1): Taq Polymerase
Taq Polymerase is the correct answer because it is the quintessential and defining enzyme of the Polymerase Chain Reaction (PCR). PCR is a revolutionary molecular technique designed to create millions of copies of a specific DNA segment *in vitro*. This multi-cycle amplification process relies on the enzyme's ability to remain active through the extremely high temperatures necessary for **DNA denaturation** (separating the double-stranded DNA template, typically at 94–98°C). Most DNA polymerases from non-thermophilic organisms would be permanently destroyed at this temperature, halting the reaction after the first cycle. *Taq* Polymerase, isolated from the heat-loving bacterium *Thermus aquaticus*, is a **thermostable DNA polymerase** that can survive these denaturation steps, making it possible for the repetitive cycles of extension to proceed and thus enabling the exponential amplification of the target DNA sequence.

The incorrect option is (2): Ribonuclease
Ribonuclease (RNase) is a class of enzymes that catalyze the degradation of RNA molecules by breaking the phosphodiester bonds within the RNA strand. This enzymatic activity is crucial for RNA processing, maturation, and the natural breakdown (turnover) of RNA within a cell. In the context of PCR, the primary goal is **DNA synthesis** and amplification, not the breakdown of nucleic acids. Therefore, a Ribonuclease would not be able to perform the function of extending the primer and creating a new complementary DNA strand, which is the central enzymatic role in the PCR process.

The incorrect option is (3): RNA Polymerase
RNA Polymerase is the enzyme responsible for **transcription** in living cells, where it synthesizes an RNA molecule (like mRNA) from a DNA template. Its functional product is RNA, not DNA. Since the Polymerase Chain Reaction is a process of **DNA replication** *in vitro*, its successful execution requires a DNA-dependent **DNA** polymerase, which builds a DNA molecule from a DNA template. An RNA Polymerase cannot utilize deoxyribonucleotides to build a DNA chain and is thus unable to mediate the amplification of DNA fragments that defines the PCR technique.

The incorrect option is (4): Endonuclease
An Endonuclease is an enzyme that cleaves the phosphodiester bonds within a DNA or RNA molecule, breaking the nucleic acid chain into fragments. The most common examples, the **Restriction Endonucleases**, are used to cut DNA at very specific recognition sites, which is vital for cloning and recombinant DNA technology. However, the fundamental role of the enzyme in the PCR cycle is **synthesis** (the Extension step), where new phosphodiester bonds are created to lengthen the DNA strand. While nucleases have numerous applications in molecular biology, they do not possess the necessary polymerase activity to achieve the amplification required by PCR.