Question

PCR of G-C rich regions (GC content $>$60%) pose some of the greatest challenges to PCR. However, this problem can be overcome by using which of the following?

• A. Optimizing the concentration of Magnesium chloride
• B. Increasing the concentration of Taq polymerase
• C. Using betaine or 7-deaza-2'-deoxyguanosine
• D. Increasing the initial denaturation time

Hint:

Adding $5\%$ to $10\%$ DMSO or $1\text{ M}$ to $2\text{ M}$ betaine is a cost-effective way to improve PCR amplification of GC-rich templates without redesigning primers.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Polymerase Chain Reaction (PCR) amplification of DNA templates with a high GC content ($>60\%$) is often difficult due to the high thermal stability of G-C base pairs.
G-C base pairs share three hydrogen bonds, making them harder to denature than A-T base pairs, which share only two.
This high stability can lead to incomplete strand separation during the denaturation step and the formation of stable secondary structures, such as hairpins, in both the template and primers.

Step 2: Detailed Explanation:
Stable secondary structures block the progression of Taq DNA polymerase along the template, leading to truncated PCR products or no amplification at all.
To solve this issue, chemical additives called PCR cosolvents or denaturing agents are added to the reaction mixture.
Betaine (trimethylglycine) acts as an osmoprotectant and iso-stabilizing agent.
It alters the hydration shell of the DNA double helix, reducing the thermal stability of G-C base pairs and equalizing the melting temperatures of A-T and G-C pairs.
This structural change allows the DNA template to denature at lower temperatures and prevents hairpin formation.
7-deaza-2'-deoxyguanosine ($\text{dc}^7\text{GTP}$) is a modified nucleotide analog that can be incorporated by DNA polymerase in place of dGTP.
The modified guanosine analog forms only two hydrogen bonds with cytosine instead of three.
This weaker base pairing destabilizes any newly synthesized secondary structures, allowing the polymerase to read through GC-rich regions.
Other common additives include dimethyl sulfoxide (DMSO) and formamide, which also destabilize the DNA double helix.

Step 3: Final Answer:
The challenge of amplifying GC-rich regions is resolved by using betaine or 7-deaza-2'-deoxyguanosine.