Q33. Which one of the following enzymes is used in Polymerase Chain Reaction ?
(A) Klenow fragment
(B) Taq polymerase
(C) T7 polymerase
(D) Primase
Taq polymerase is the enzyme used in Polymerase Chain Reaction (PCR). This thermostable DNA polymerase from Thermus aquaticus enables repeated cycles of DNA denaturation, annealing, and extension without degrading.
Option Analysis
Klenow fragment derives from E. coli DNA polymerase I and lacks thermostability, making it unsuitable for PCR’s high temperatures; it serves in DNA labeling and fill-in reactions.
Taq polymerase withstands 95°C denaturation, synthesizes DNA at 72°C (about 1000 bases/minute), and drives exponential amplification across 25-40 cycles.
T7 polymerase (T7 RNA polymerase) transcribes DNA to RNA and requires specific promoters; it does not perform DNA-dependent DNA synthesis for PCR.
Primase synthesizes short RNA primers during natural replication but cannot extend DNA strands or endure PCR heat cycles.
Introduction to PCR and Key Enzyme
Polymerase Chain Reaction (PCR) amplifies specific DNA segments through thermal cycling: denaturation (95°C), annealing (50-65°C), and extension (72°C). The enzyme used in Polymerase Chain Reaction must survive high heat—Taq polymerase fits perfectly due to its thermostability from Thermus aquaticus.
Why Taq Polymerase Excels in PCR
Taq polymerase binds primers, adds dNTPs via 5’→3′ synthesis, and achieves high processivity without replenishment per cycle. Its half-life exceeds 40 minutes at 95°C, enabling reliable amplification for cloning, diagnostics, and forensics.
-
Heat stability prevents denaturation during cycles
-
Optimal activity at 72°C with 150 nucleotides/second speed
-
No 3’→5′ proofreading, but sufficient for standard PCR
Comparing PCR Enzymes: Why Not Others?
| Enzyme | Source/Function | PCR Suitability Reason |
|---|---|---|
| Klenow fragment | E. coli DNA Pol I fragment | Heat-labile; used for labeling |
| Taq polymerase | Thermus aquaticus | Thermostable; core PCR enzyme |
| T7 polymerase | T7 bacteriophage | RNA synthesis, not DNA extension |
| Primase | RNA primer synthesis | Heat-sensitive; short primers only |
Taq polymerase remains the standard for CSIR NET questions on PCR enzymes.
CSIR NET Exam Insights
For competitive exams like CSIR NET Life Sciences, recognize Taq polymerase as the answer to “which one of the following enzymes is used in Polymerase Chain Reaction?” Options test thermostability knowledge—eliminate non-DNA polymerases or heat-unstable ones.
