Question
Explain the principle and steps of Polymerase Chain Reaction (PCR). Why is Taq polymerase used? List important applications of PCR.
(NCERT Class 12, high-frequency NEET question)
Solution — Step by Step
PCR is an in vitro technique to amplify a specific DNA segment millions of times in a few hours. It mimics DNA replication but uses repeated heating and cooling cycles to denature and re-synthesise DNA.
Invented by Kary Mullis in 1983 (Nobel Prize, 1993). Think of PCR as a molecular photocopier for DNA.
- Template DNA: The DNA containing the target sequence to be amplified.
- Two primers: Short, synthetic oligonucleotides (18-25 bases) complementary to the flanking regions of the target sequence. One for each strand.
- Taq DNA polymerase: A thermostable DNA polymerase isolated from Thermus aquaticus (a bacterium from hot springs). It can withstand the high temperatures used in PCR.
- dNTPs: Deoxynucleotide triphosphates (dATP, dTTP, dGTP, dCTP) — the building blocks.
- Buffer with : Required for polymerase activity.
Each cycle has three temperature-controlled steps:
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Denaturation (94-98 degrees C, ~30 seconds): The double-stranded DNA is heated to separate (denature) into two single strands by breaking hydrogen bonds.
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Annealing (50-65 degrees C, ~30 seconds): The temperature is lowered so that the primers bind (anneal) to their complementary sequences on each single-stranded template. The annealing temperature depends on the primer sequence.
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Extension (72 degrees C, ~1 minute): Taq polymerase synthesises new DNA strands starting from each primer, using dNTPs. It extends in the 5’ to 3’ direction, using the original strands as templates.
After one cycle: 2 copies. After cycles: copies. After 30 cycles: over 1 billion copies of the target DNA.
Normal DNA polymerases (like E. coli DNA Pol I) would be destroyed at 94 degrees C during the denaturation step. Taq polymerase is thermostable — it functions optimally at 72 degrees C and survives repeated heating to 94 degrees C. This eliminates the need to add fresh enzyme after each cycle, making the process automatable in a thermocycler.
Why This Works
PCR works because DNA denaturation and renaturation are reversible, temperature-dependent processes. By cycling between high temperature (denature) and low temperature (anneal + extend), we can repeatedly copy the same DNA segment. Exponential amplification occurs because each newly synthesised strand becomes a template in the next cycle.
The specificity comes from the primers — they determine exactly which segment of the genome is amplified. Everything else in the genome is ignored.
Applications of PCR (NEET favourites): DNA fingerprinting, diagnosis of genetic diseases, detection of HIV and other pathogens, forensic analysis from crime scene DNA, paternity testing, and amplification of DNA for cloning and sequencing.
Common Mistake
Students often write the three steps in the wrong order or mix up the temperatures. The correct order is: Denaturation (94 degrees C) → Annealing (50-65 degrees C) → Extension (72 degrees C). A common error is placing extension before annealing — but primers must bind first before polymerase can extend.
Another mistake: saying PCR can amplify RNA directly. PCR works only on DNA. To amplify RNA (e.g., from an RNA virus), you must first convert it to cDNA using reverse transcriptase — this modified technique is called RT-PCR (Reverse Transcriptase PCR). Do not confuse RT-PCR with real-time PCR (qPCR), which quantifies DNA during amplification.