Q.53 To obtain a large number of copies of DNA through PCR includes the following steps.
- Double stranded DNA with known ends.
- Separate chains by heating to 95°C.
- Addition of heat tolerant – Taq polymerase and nucleotides and heat to 72°C.
- Addition of nucleotides to primer using single strand as template.
- Cool to allow primer to form hydrogen bonds with ends of target sequence.
Choose the correct answer from the options given below :
- A, B, D, C, E
- A, B, C, D, E
- A, B, E, C, D
- A, B, D, E, C
PCR amplifies DNA through a cyclic process of denaturation, annealing, and extension, and the correct sequence of the given steps is A, B, E, C, D.
Step Explanations
PCR requires double-stranded DNA (dsDNA) with known end sequences for primer design (Step A). Heating to 95°C denatures dsDNA into single strands by breaking hydrogen bonds (Step B). Cooling then allows primers to anneal via hydrogen bonding to complementary target sequences (Step E, typically 50-65°C). Next, Taq polymerase (heat-stable from Thermus aquaticus) and dNTPs are present, with heating to 72°C for extension, where nucleotides add to the primer using the single strand as template (Steps C and D).
Option Analysis
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A, B, D, C, E: Incorrect; places extension (D/C) before annealing (E), which fails without primers bound.
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A, B, C, D, E: Incorrect; attempts extension (C/D) before annealing (E), skipping primer binding.
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A, B, E, C, D: Correct; follows denaturation (B), annealing (E), then extension (C/D).
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A, B, D, E, C: Incorrect; premature extension (D) before annealing (E).
Introduction to PCR Steps Sequence
The PCR steps sequence is essential for amplifying a large number of DNA copies in molecular biology, particularly for GATE Life Sciences exams. This technique uses denaturation, annealing, and extension cycles with Taq polymerase to produce millions of target DNA copies from dsDNA with known ends.
Detailed PCR Steps Sequence
PCR begins with double-stranded DNA with known ends (A), enabling specific primer design.
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Separate chains by heating to 95°C (B): Denatures dsDNA into single strands.
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Cool to allow primer annealing (E): Primers bind via hydrogen bonds at 50-65°C.
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Add Taq polymerase, nucleotides, heat to 72°C (C): Enables extension.
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Nucleotides add to primer on template (D): Synthesizes new strands.
These repeat 20-40 cycles for exponential amplification.
Step Label Description Temperature Purpose A dsDNA with known ends Initial setup Template preparation B Denaturation 95°C Strand separation E Annealing 50-65°C Primer binding C Add Taq/dNTPs, extension 72°C Polymerization start D Nucleotide addition 72°C Strand synthesis GATE Q.53 Solution
For “To obtain a large number of copies of DNA through PCR,” the correct order is A, B, E, C, D. This matches the standard cycle: setup, denature, anneal, extend. Other options disrupt this by misplacing annealing or extension.
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