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Week 6 Lab: Gibson assembly

Week 6 — Genetic Circuits Part I

Protocol: DNA Assembly

1. What are some components in the Phusion High-Fidelity PCR Master Mix, and what is their purpose?

The Phusion High-Fidelity PCR Master Mix contains several components required for efficient and accurate DNA amplification. The key components are:

• Phusion DNA Polymerase: A high-fidelity thermostable polymerase derived from Pyrococcus. It has proofreading (3'→5' exonuclease) activity, which reduces the error rate during DNA synthesis
• dNTPs (deoxynucleotide triphosphates): These are the building blocks used by the polymerase to synthesize the new DNA strand
• Reaction buffer: Provides optimal ionic strength and pH for polymerase activity. It usually contains Mg²⁺, which is required as a cofactor for DNA polymerase
• MgCl₂: Magnesium ions stabilize primer-template interactions and are essential for the catalytic activity of the polymerase
• Stabilizers and additives: These help maintain enzyme stability and improve amplification efficiency

The master mix in PCR is prepared to simplify the reaction setup and ensure consistency across multiple reactions. Instead of adding each component separately, the master mix contains the essential reagents required for DNA amplification, such as the polymerase, buffer, Mg²⁺ ions, and dNTPs.

In practice, the total master mix volume is calculated based on the number of reactions to be performed, and it is recommended to prepare additional volume (usually enough for one or two extra reactions) to account for a negative control and possible pipetting errors.

In general, using a master mix also helps reduce variability between reactions and ensures high specificity and low mutation rates during amplification when using high-fidelity enzymes such as Phusion polymerase.

1. What are some factors that determine primer annealing temperature during PCR?

The primer annealing temperature (Ta) is critical for PCR specificity and efficiency. Some factors that determine the annealing temperature include:

1. Primer melting temperature (Tm): The annealing temperature is typically 3–5°C below the primer Tm
2. Primer length: Longer primers generally have higher melting temperatures
3. GC content: Primers with higher GC content have stronger hydrogen bonding and therefore higher Tm