Question

Which of the following parameters affect the annealing of primer to the template in PCR?
(A) Nucleotide sequence of the primer.
(B) Temperature of annealing.
(C) Specificity of Taq DNA Polymerase.
(D) GC content of the template.
(E) Concentration of $\text{Mg}^{2+}$ ions.
Choose the correct answer from the options given below:

• A. (A), (B) and (D) only.
• B. (A), (B), (D) and (E) only.
• C. (A), (B), (C) and (D) only.
• D. (B), (D) and (E) only.

Hint:

Optimal magnesium concentration is crucial:
Too little $\text{Mg}^{2+}$ $\rightarrow$ Weak primer-template hybridization (poor yield).
Too much $\text{Mg}^{2+}$ $\rightarrow$ Over-stabilizes non-specific hybridization (non-specific bands).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The annealing step of the Polymerase Chain Reaction (PCR) involves the physical hybridization of synthetic single-stranded oligonucleotide primers to their complementary target sequences on the denatured single-stranded template DNA.
This hybridization process is governed by thermodynamic stability and chemical kinetics.
Detailed Explanation:
Let us analyze each parameter:
- (A) Nucleotide sequence of the primer: This directly determines the degree of complementarity between the primer and the template, and dictates the melting temperature ($T_m$). Thus, (A) affects annealing.
- (B) Temperature of annealing ($T_a$): The annealing temperature must be optimized (usually $3\text{ to }5^\circ\text{C}$ below the primer $T_m$).
If too high, the primer cannot hybridize; if too low, non-specific mismatch annealing occurs. Thus, (B) affects annealing.
- (D) GC content of the template/primer: G-C base pairs contain three hydrogen bonds, making them thermodynamically more stable than A-T base pairs.
A higher GC content increases the melting temperature, directly influencing the annealing dynamics. Thus, (D) affects annealing.
- (E) Concentration of $\text{Mg^{2+}$ ions:} Magnesium ions act as divalent cations that shield the negatively charged phosphate backbones of the DNA strands, reducing electrostatic repulsion and stabilizing the primer-template hybrid. Thus, (E) affects annealing.
- (C) Specificity of Taq DNA Polymerase: Taq polymerase acts during the extension/elongation phase. It does not influence the physical hybridization of the primer to the template during the annealing step.
Therefore, parameters (A), (B), (D), and (E) directly affect the annealing process.

Step 2: Final Answer:
The correct option is (B).