Week 06 - Genetic Circuits

  1. Lab Protocol: PCR, Digestion, and Assembly Strategies1. Phusion High-Fidelity PCR Master MixPhusion is the standard for cloning due to its high speed and accuracy.ComponentPurposePhusion DNA PolymeraseA “Pyrococcus-like” enzyme fused to a dsDNA-binding domain. This provides extreme processivity and 3’→5’ exonuclease activity (proofreading) for high fidelity.dNTPsDeoxynucleotide triphosphates ($A, T, C, G$)—the raw building blocks for DNA synthesis.Phusion HF (or GC) BufferMaintains optimal pH and provides $MgCl_2$ cofactors essential for polymerase activity.Hot Start AdditivesReversible inhibitors that prevent non-specific amplification at room temperature.

  2. Factors Determining Primer Annealing Temperature ($T_a$)The $T_a$ must be optimized to ensure primers bind specifically to the target.Primer Melting Temperature ($T_m$): Calculated based on length and GC content (higher GC = higher $T_m$).Salt Concentration: The high salt in Phusion buffers stabilizes DNA duplexes; $T_a$ is usually set 3°C higher than the calculated $T_m$.Primer Concentration: Excess primers can slightly increase the effective $T_m$.Mismatches/Overhangs: For cloning, only the region perfectly binding to the template determines the $T_a$ for the initial cycles.

  3. Comparison: PCR vs. Restriction Enzyme DigestWhile both create linear DNA, they are used for different tactical purposes.FeaturePCR (Polymerase Chain Reaction)Restriction Enzyme DigestMechanismSynthesis of new DNA copies from a template.Mechanical “cutting” of existing DNA at specific sites.YieldHigh. Exponentially amplifies the target.Low. Limited by the starting material quantity.End ResultUsually blunt ends (with Phusion). Allows for custom overhangs.Sticky or blunt ends depending on the specific enzyme.AccuracyHigh, but carries a small risk of point mutations.Near-perfect, as it only isolates existing sequences.

  4. Ensuring Suitability for Gibson AssemblyTo ensure your products are “Gibson-ready,” confirm the following:Overlaps: Adjacent fragments must share 15–40 bp of identical sequence.No Secondary Structure: Check that overlap regions do not form stable hairpins or dimers.Purification: PCR products must be purified to remove polymerase, which can interfere with the 5’ “chewing-back” process.Directionality: Ensure primers are oriented so fragments assemble in the correct $5’ \rightarrow 3’$ order.

  5. How DNA Enters E. coliDuring chemical transformation (Heat Shock):Calcium Ions ($Ca^{2+}$): $CaCl_2$ coats the negatively charged DNA and the bacterial cell wall, neutralizing repulsive forces.Heat Pulse ($42^\circ\text{C}$): A sudden temperature jump creates a pressure imbalance that forces “pores” to open in the membrane.Membrane Depolarization: The heat shock decreases the membrane potential, allowing DNA to cross into the cytoplasm.

  6. Alternative Assembly: Golden Gate (GGA)Golden Gate Assembly is a modular technique using Type IIS restriction enzymes (e.g., BsaI). These enzymes cut outside their recognition sites, creating unique 4-bp sticky overhangs. This allows multiple fragments to be assembled in a specific order in a single “one-pot” reaction. The process cycles between digestion and ligation; once fragments are correctly ligated, the recognition sites are removed, driving the reaction toward the final circular product.