BiotechnologyHigh NEET PriorityClass 12
PCR
Definition
PCR (Polymerase Chain Reaction) is a laboratory technique used to amplify specific DNA segments, creating millions of copies from a small sample. Invented by Kary Mullis in 1983, PCR uses thermal cycling with a heat-stable DNA polymerase (Taq polymerase) and specific primers.
Key Points for NEET
- 1Three steps: Denaturation (94°C), Annealing (55-65°C), Extension (72°C)
- 2Uses Taq polymerase (from Thermus aquaticus)
- 3Requires primers, dNTPs, and template DNA
- 4Exponential amplification: 2^n copies after n cycles
- 5Applications: diagnosis, forensics, research
Example
COVID-19 RT-PCR test amplifying viral RNA to detect infection
Asked in NEET
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Common Mistakes to Avoid
- ✗Confusing the 3 steps: denaturation (separate strands) ≠ annealing (primers bind) ≠ extension (Taq adds nucleotides)
- ✗Thinking any DNA polymerase works in PCR — only Taq polymerase survives the 94°C denaturation step
- ✗Forgetting PCR needs TWO primers (forward and reverse) flanking the target sequence
Quick Revision Notes
- ⚡3 steps per cycle: Denaturation (94°C) → Annealing (55-65°C) → Extension (72°C); ~30 cycles = 10⁹ copies
- ⚡Taq polymerase from Thermus aquaticus (hot springs bacterium) — thermostable, works at 72°C
- ⚡Applications: DNA fingerprinting, disease diagnosis, forensics, paternity testing, gene cloning
- ⚡RT-PCR: reverse transcriptase first converts RNA → cDNA, then PCR amplifies (used for COVID testing)
Related Terms
DNA AmplificationTaq PolymerasePrimersThermal Cycler
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