Q.16 Arrange the given steps involved in polymerase chain reaction-
(A). Denaturation
(B). Annealing
(C). Extension
(D). Cycling
Answer: (A) → (B) → (C) → (D) (Denaturation → Annealing → Extension → Cycling).
PCR follows the standard cycle: denaturation (94-98°C, strand separation), annealing (50-65°C, primer binding), extension (72°C, Taq polymerase synthesis), repeated through cycling (20-40 cycles).
Option Analysis
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(A) Denaturation: First step (94-98°C, 30s); heat separates DNA double helix into single strands for template access.
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(B) Annealing: Second step (50-65°C, 30s); primers hybridize to complementary sequences on single-stranded templates.
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(C) Extension: Third step (72°C, 1-2 min/kb); Taq DNA polymerase extends primers using dNTPs, synthesizing new strands.
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(D) Cycling: Final process; entire A-B-C sequence repeated 20-40 times via thermal cycler for exponential amplification (2ⁿ copies).
Logical Sequence: A (template prep) → B (primer binding) → C (DNA synthesis) → D (repetition).
PCR steps sequence denaturation annealing extension cycling forms the foundation of GATE Life Sciences molecular biology syllabus. Standard order: (A) Denaturation → (B) Annealing → (C) Extension → (D) Cycling.
Complete PCR Cycle Parameters
| Step | Temperature | Duration | Purpose |
|---|---|---|---|
| (A) Denaturation | 94-98°C | 30s | dsDNA → ssDNA templates |
| (B) Annealing | 50-65°C | 30s | Primer hybridization |
| (C) Extension | 72°C | 1 min/kb | Taq polymerase synthesis |
| (D) Cycling | 20-40 cycles | 2-3 hrs total | Exponential amplification |
Temperature Cycling Profile
Initial denaturation: 95°C, 5 min
↓
[Cycle 1-30]:
A: 95°C, 30s → B: 55°C, 30s → C: 72°C, 1 min
↓
Final extension: 72°C, 5 min
GATE Life Sciences Memory Aid
“DAE-Cycle”: Denature (hot), Anneal (cool), Extend (warm), Cycle (repeat). Taq polymerase from Thermus aquaticus survives 95°C denaturation.
Pro tip: Links perfectly to your colony hybridization series—PCR amplifies target DNA, colony hybridization screens bacterial clones containing that sequence.
Master PCR steps sequence = guaranteed molecular biology scores in competitive exams.
