Question

During PCR, primers bind to the DNA strands in the __________ step.

• A. ligation
• B. denaturation
• C. extension
• D. annealing

Hint:

• Primer Design: Primers are designed in pairs (forward and reverse) to flank the region of interest. They are oriented such that DNA synthesis proceeds inward across the target sequence.

• Thermostability: Taq polymerase is isolated from the hot-spring bacterium Thermus aquaticus. This enzyme can withstand the repeated high-temperature denaturation steps without denaturing itself, which is essential for automating the PCR cycle.

Answer 1

The Correct Option is D

Concept:

• Polymerase Chain Reaction (PCR): An enzymatic, in-vitro technique used to amplify a specific target sequence of DNA into millions of copies.

• A single cycle of PCR involves three distinct temperature-dependent steps: itemize

• Denaturation: The double-stranded template DNA is heated (typically to \(94-96^\circ\text{C}\)) to disrupt hydrogen bonds, separating the DNA into single strands.

• Annealing: The temperature is lowered (typically to \(50-65^\circ\text{C}\)) to allow synthetic oligonucleotide primers to bind (hybridize) to their complementary sequences on the single-stranded template DNA.

• Extension: The temperature is adjusted (typically to \(72^\circ\text{C}\)) to allow a thermostable DNA polymerase (such as Taq polymerase) to extend the primers by adding complementary deoxynucleotide triphosphates (dNTPs).

Ligation: This is a distinct process of joining DNA fragments catalyzed by DNA ligase, which is not a part of the standard PCR cycle. itemize

Step 1: Analyze the biochemical requirements for primer binding
Primers are short, single-stranded sequences of nucleic acids that serve as starting points for DNA synthesis. For primers to bind to the template, the double-stranded genomic DNA must first be separated into single strands via heat. Once separated, the temperature of the reaction mixture must be lowered to a point where stable hydrogen bonds can reform specifically between the primers and their complementary regions on the template strands.

Step 2: Evaluate the options against the steps of a PCR cycle

• (A) Ligation: This is a cloning step used to seal nicks in the sugar-phosphate backbone, not a component of the standard PCR amplification cycle.

• (B) Denaturation: At this high-temperature stage (\(\sim 94^\circ\text{C}\)), single-stranded DNA templates are generated, but the high kinetic energy prevents primers from annealing.

• (C) Extension: During this step (\(\sim 72^\circ\text{C}\)), DNA polymerase actively synthesizes the new strand starting from the 3'-hydroxyl end of the already-bound primers.

• (D) Annealing: This is the intermediate temperature stage (\(\sim 50-60^\circ\text{C}\)) where the thermal conditions favor the hybridization (binding) of primers to the template DNA.

Step 3: Formulate the final conclusion
The binding of synthetic primers to single-stranded template DNA occurs specifically during the annealing phase. This corresponds to option (D).