Week 6 HW: Genetic Circuits Part I: Assembly Technologies
DNA Assembly
Components in Phusion High-Fidelity PCR Master Mix and their purpose
Phusion High-Fidelity PCR Master Mix typically contains a high-fidelity DNA polymerase (such as Phusion polymerase), dNTPs, Mg²⁺ (often as MgCl₂), and a reaction buffer. The high-fidelity polymerase has proofreading (3’→5’ exonuclease) activity, which reduces DNA replication errors.
Factors determining primer annealing temperature
Primer annealing temperature depends mainly on:
- primer melting temperature (Tm), which is influenced by GC content (stronger hydrogen bonding)
- primer length
- nucleotide sequence composition
- kation concentration, such as Mg²⁺, also affects primer binding stability
- primer design factors such as self-complementarity, secondary structures (hairpins), and primer-dimer formation
PCR vs restriction enzyme digestion
| Feature | PCR | Restriction Enzyme Digestion |
|---|---|---|
| Purpose | Amplifies a specific DNA fragment | Cuts DNA at specific recognition sites |
| Mechanism | Uses primers and DNA polymerase through thermal cycling | Uses restriction enzymes that recognize and cleave specific sequences |
| Input requirement | Requires primers designed for target sequence | Requires existing DNA containing restriction sites |
| Output | Many copies of a defined DNA fragment | One or more DNA fragments of defined lengths |
| Specificity control | Determined by primer design | Determined by enzyme recognition sites in DNA |
| Equipment needed | Thermocycler | Usually incubator/water bath |
| Flexibility | High (can create custom fragments) | Limited to naturally occurring or engineered restriction sites |
| Typical use case | Amplifying genes for cloning, sequencing, diagnostics | Cutting plasmids or genomic DNA for cloning or mapping |
| Advantages | Fast amplification, highly specific, can introduce mutations or tags | Predictable cuts, simple workflow, widely used in traditional cloning |
| Limitations | Requires careful primer design, risk of amplification errors | Requires suitable restriction sites, can leave unwanted “scar” sequences |
Ensuring DNA fragments are suitable for Gibson cloning
To ensure compatibility with Gibson assembly, DNA fragments must have overlapping homologous ends (typically 20–40 bp) that match adjacent fragments. These overlaps are usually added through PCR primer design. PCR products should be purified to remove enzymes, primers, and nucleotides that could interfere with assembly. For restriction-digested fragments, ends must be designed or processed to create compatible overlaps (often via PCR or adapter sequences). Additionally, sequences should be checked for unwanted secondary structures, repeats, or mismatches in overlap regions to ensure efficient annealing and correct assembly.
How plasmid DNA enters E. coli during transformation
Plasmid DNA (outside cell) → Cell made competent (CaCl₂ treatment or electroporation) → Negative charges on DNA + cell membrane partially neutralized → DNA brought close to bacterial membrane → Heat shock (chemical method) OR electric pulse (electroporation) → Temporary pores form in membrane → Plasmid DNA enters cytoplasm → Membrane reseals → Plasmid is maintained and can replicate inside E. coli
Golden Gate Assembly
Golden Gate Assembly is a molecular cloning method that uses Type IIS restriction enzymes, such as BsaI, which cut DNA outside of their recognition sites. This creates custom-designed overhangs that allow multiple DNA fragments to be ligated together in a specific order. The reaction occurs in a single tube containing both the restriction enzyme and DNA ligase, cycling between digestion and ligation steps. Because the recognition sites are removed during assembly, the final construct is seamless and does not retain unwanted restriction sequences. This method is highly efficient for assembling multiple fragments simultaneously, making it useful for constructing complex genetic circuits or multi-gene plasmids. It is often preferred over Gibson assembly when precise, standardized overhangs are desired and when high-throughput cloning is needed.