week 6 genetic circuits part I'

basic Concepts

This week we learn core molecular biology tools and techniques for processing and assembling DNA, including PCR and Gibson Assembly.

1. Components of Phusion High-Fidelity PCR Master Mix

Phusion Master Mix contains several key components:

• Phusion Hot Start II DNA Polymerase — A high-fidelity polymerase with a proofreading (3’→5’ exonuclease) domain that corrects misincorporated bases, resulting in ~50× lower error rates than Taq. It also has a processivity-enhancing domain that speeds up elongation.
• dNTPs (dATP, dCTP, dGTP, dTTP) — The nucleotide building blocks incorporated during strand synthesis.
• MgCl₂ — Magnesium ions are an essential cofactor for DNA polymerase activity and also stabilize the dNTP substrates.
• Optimized reaction buffer — Maintains proper pH and ionic conditions for polymerase activity and primer/template annealing.
• Stabilizers/additives — Help maintain enzyme stability and can improve yield on difficult templates (e.g., GC-rich regions).

2. Factors Determining Primer Annealing Temperature

• Primer GC content — G·C pairs form 3 hydrogen bonds vs. 2 for A·T, so higher GC content raises the melting temperature (Tm). A rough formula is Tm = 4(G+C) + 2(A+T).
• Primer length — Longer primers have higher Tm values because more base-pair interactions must be disrupted.
• Salt/ion concentration — Higher Mg²⁺ or monovalent salt concentrations stabilize the DNA duplex and raise Tm.
• Primer secondary structure — Hairpins or self-dimers can reduce effective annealing efficiency.
• Template secondary structure — Highly structured templates may require higher annealing temperatures or additives like DMSO.
• Mismatches — Deliberate mismatches (e.g., for mutagenesis) lower Tm and require adjusted annealing temperatures.
• Annealing temperature rule of thumb — Typically set 5°C below the lower Tm of the two primers used.

3. PCR vs. Restriction Enzyme Digests

When to prefer PCR

• You need to amplify a fragment from a complex mixture (e.g., genomic DNA).
• You want to add sequences (overhangs, restriction sites, Gibson overlaps) to the ends of a fragment.
• No convenient restriction sites flank your gene of interest.
• You are introducing a point mutation or modifying a sequence.

When to prefer restriction enzyme digest

• You are sub-cloning between two vectors that already have compatible restriction sites.
• You need sticky ends for directional cloning.
• You want to cut a vector backbone without amplifying it (avoids PCR errors in the vector).
• Speed and simplicity are priorities when restriction sites are already present.

4. Ensuring Compatibility with Gibson Assembly

Gibson Assembly requires fragments with overlapping homologous sequences (~15–30 bp) at their ends. To ensure compatibility:

• For PCR fragments: Design primers so that the 5’ overhang of each primer matches the end of the adjacent fragment. This way, after PCR, the amplified insert carries ~20–30 bp of sequence identical to the neighboring fragment or vector.
• For restriction-digested fragments: After digestion, check that the sticky ends or blunt ends are located within the overlap region you plan to use — or add Gibson overlaps via a subsequent PCR step using primers that extend into the adjacent sequence.
• Check orientation: Use Benchling or SnapGene to simulate the assembly and verify that all overlaps are in the correct orientation and reading frame.
• Avoid internal repeat sequences in the overlap regions, as the exonuclease in Gibson mix can cause misannealing.
• Ensure no unwanted restriction sites or stop codons are introduced at junctions.
• Gel-purify or column-purify fragments after PCR or digest to remove enzymes, primers, and small fragments that could interfere.

5. How Plasmid DNA Enters E. coli During Transformation

The most common method in lab courses is heat-shock transformation of chemically competent cells:

1. Chemical competency preparation — Cells are treated with divalent cations (typically CaCl₂), which neutralize the negative charges on the LPS of the outer membrane and on the DNA, reducing electrostatic repulsion.
2. DNA binding — Plasmid DNA associates with the cell surface, facilitated by the Ca²⁺ ions.
3. Heat shock (42°C, ~30–45 sec) — The rapid temperature increase is thought to create a thermal imbalance that momentarily destabilizes the membrane and drives DNA into the cell, possibly through transient pores or membrane disruptions. The exact mechanism is still not fully understood.
4. Recovery on ice — Cells are rapidly cooled to stabilize the membrane after DNA entry.
5. Outgrowth in SOC/LB — Cells recover and begin expressing antibiotic resistance genes before plating on selective media.

Alternative method — Electroporation: A brief electrical pulse (~1.8–2.5 kV) creates transient pores in the membrane through which DNA passes. This is more efficient but requires electrocompetent cells and specialized equipment.

6. Golden Gate Assembly

6.1 Explanation in 5–7 sentences

Golden Gate Assembly is a DNA assembly technique that uses Type IIS restriction enzymes, such as BsaI or BsmBI, which cut outside of their recognition sites rather than within them. This makes it possible to design custom overhangs that determine the exact order in which DNA fragments join together. In a single reaction tube, the restriction enzyme cuts the DNA fragments and vector, and DNA ligase joins the matching overhangs. Because the recognition sites can be removed during the assembly process, the final DNA construct is often scarless, meaning no extra unwanted sequence remains at the junctions. Golden Gate Assembly is especially useful for assembling multiple DNA fragments in a defined order with high efficiency. It is widely used in modular cloning systems and synthetic biology workflows. Compared with Gibson Assembly, Golden Gate relies on restriction sites and short designed overhangs rather than long homologous overlaps.

6.2 Simple diagram

Resources

• Primer Design: HTGAA’s Supplement to Gibson Assembly Recitation
• NEB’s (New England Biolabs) video Introduction to Gibson Assembly
• NEB’s (New England Biolabs) explanation & protocols for Gibson Assembly®

General principle

Fragment 1      Fragment 2      Fragment 3
  [BsaI]          [BsaI]          [BsaI]
     |               |               |
     v               v               v
Cut outside the recognition sequence to create custom overhangs

Overhangs designed as:
Fragment 1 ---> AATG
Fragment 2 ---> GCTT
Fragment 3 ---> CGGA

Matching overhangs guide ligation in the correct order:

Fragment 1 + Fragment 2 + Fragment 3
        ↓
Final assembled construct

1. Type IIS restriction enzyme cuts DNA outside its recognition site
2. Custom sticky ends are generated
3. Matching sticky ends anneal
4. DNA ligase seals the backbone
5. Final construct forms without the original restriction sites