Week 6 HW: Genetic Circuits: Part I
Week 6 — Genetic Circuits Part I: Assembly Technologies
DNA Assembly
Answer these questions about the protocol in this week’s lab:
- 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:
- Phusion DNA polymerase → a high-fidelity enzyme that synthesizes DNA with very low error rates (With a failure rate 50 times lower than Taq and 6 times lower than Pfu, these polymerases are an excellent choice for cloning and other applications requiring high fidelity), which is critical when amplifying fragments of the amilCP gene.
- dNTPs (deoxynucleotide triphosphates) → building blocks for new DNA strands
- MgCl₂ → cofactor necessary for polymerase activity
- Buffer system → maintains optimal pH and ionic conditions These components work together to ensure accurate and efficient DNA amplification, also Phusion DNA polymerases offer robust performance with short protocol times, even in the presence of PCR inhibitors. They generate higher yields with less enzyme than other DNA polymerases. In this protocol, the master mix is used to amplify amilCP fragments that will later be assembled using Gibson Assembly.
- What are some factors that determine primer annealing temperature during PCR? Primer annealing temperature depends on:
- Primer length → longer primers have higher melting temperatures,
- GC content → higher GC increases stability and raises Tm. Higher melting temperatures are caused due to stronger hydrogen bonding. In this protocol, primers include additional overhangs (20–22 bp) for Gibson Assembly, but only the binding region determines the annealing temperature. The annealing temperature is typically set a few degrees below the melting temperature (Tm) to ensure specific binding.
There are two methods from this class that create linear fragments of DNA: PCR, and restriction enzyme digests. Compare and contrast these two methods, both in terms of protocol as well as when one may be preferable to use over the other. In this protocol, PCR amplify specific regions of the amilCP gene, including mutated regions in the chromophore, allowing precise control over sequence design In contrast, restriction digestion (using PvuII) is used to linearize the pUC19 plasmid backbone. PCR is more flexible and allows introduction of mutations and overlaps, while restriction digestion relies on specific enzyme recognition sites. PCR is preferable for designing new constructs, whereas digestion is useful for preparing existing plasmid backbones.
How can you ensure that the DNA sequences that you have digested and PCR-ed will be appropriate for Gibson cloning? To ensure compatibility with Gibson Assembly, DNA fragments must have overlapping homologous regions of ～20–22 base pairs. In this protocol, these overlaps are introduced through primer design during PCR amplification of the amilCP fragments. The pUC19 backbone generated by restriction digestion also contains compatible ends. These overlaps allow fragments to anneal and be joined seamlessly during the Gibson Assembly reaction.
How does the plasmid DNA enter the E. coli cells during transformation? Plasmid DNA enters E. coli cells during transformation through heat shock or electroporation. In heat shock, cells are chemically treated (for example with CaCl₂) and briefly heated, creating pores in the membrane In electroporation, an electric pulse temporarily disrupts the membrane These methods allow DNA to pass into the cell, where it can replicate. Once inside, the plasmid replicates and expresses the amilCP gene, allowing colonies to be visually identified by color.
Describe another assembly method in detail (such as Golden Gate Assembly) a) Explain the other method in 5 - 7 sentences plus diagrams (either handmade or online). Golden Gate Assembly is a molecular cloning method that uses Type IIS restriction enzymes (such as BsaI) and DNA ligase in a single reaction. These enzymes cut DNA outside their recognition site, generating customizable overhangs. This allows multiple DNA fragments to be assembled in a specific order without leaving unwanted sequences (scarless assembly). The reaction cycles between digestion and ligation, increasing efficiency. Because of its precision, Golden Gate is ideal for assembling multiple fragments simultaneously. It is widely used in synthetic biology for modular cloning. Compared to Gibson Assembly, it relies more on restriction sites rather than homologous overlaps. 
- Create a blank Construct and save it to your Repository a) Recreate the Repressilator in that empty Construct by using parts from the Characterized Bacterial Parts repository b) Search the parts using the Search function in the right menu c) Drag and drop the parts into the Construct d) Confirm it works as expected by running the Simulator (“play” button) and compare your results with the Repressilator Construct found in the Bacterial e) Demos repository f)Document all of this work in your Notebook entry - you can copy the glyph image and the simulator graphs, and paste them into your Notebook
- Build three of your own Constructs using the parts in the Characterized Bacterials Parts Repo a) Explain in the Notebook Entry how you think each of the Constructs should function b) Run the simulator and share your results in the Notebook Entry c) If the results don’t match your expectations, speculate on why and see if you can adjust the simulator settings to get the expected outcome
I got an error in Kernel so I will upload here the information
The focus of this investigation is the Repressilator, a prototypical synthetic genetic circuit, which demonstrates the implementation of a negative feedback loop using three transcriptional repressors (LacI, Lambda cI, and TetR) arranged in a cyclic inhibition network. The core objective of this design is to achieve sustained oscillatory behavior in gene expression. This behavior is emergent, meaning it arises solely from the interactions between the genetic parts, highlighting the power of modular design in biotechnology.
In this document, I present the recreation of this circuit using the Asimov/Kernel environment, alongside an analysis of how transcriptional parameter variations—specifically promoter strength—impact the dynamical stability of the system. This study serves as a critical prerequisite for my ongoing final project idea, Bio-Shield, where precise temporal control of biosensors is essential for reliable environmental monitoring.
The implementation of the J23100 constitutive promoter within the Repressilator architecture resulted in the complete abolition of oscillatory behavior, leading the system to settle into a stable steady state. This loss of functionality confirms that the emergence of synthetic genetic oscillators is strictly dependent on the kinetic symmetry of the circuit nodes. The high transcription flux driven by the J23100 promoter creates a metabolic imbalance that overrides the necessary negative feedback loop, preventing the sequential repression required for periodic gene expression. This experiment highlights that in synthetic biology, design modularity is not sufficient for function; fine-tuning the relative strengths of individual components is critical to maintaining the precise dynamical parameters required for complex behaviors like oscillation.
For the second construct, I implemented a NOT gate using the AmilCP chromoprotein as a visual reporter. This choice is based on the protein’s ability to provide a clear, naked-eye readout, which is highly relevant for the development of the Bio-Shield mining safety biosensor (my final project idea). The simulation confirms that the construct acts as a genetic switch: when the repression signal is removed, AmilCP accumulates, resulting in a visible blue output.
The final construct demonstrates the successful functional coupling of a threshold-logic biosensor to the core Repressilator oscillator. By connecting the TetR-repressible pTetR promoter to the LacI node, the biosensor’s activation is now gated by the central clock. This hierarchical architecture ensures that the safety alert system (gated by LacI repression) only triggers when the environmental input threshold is met, but only during specific phases of the cellular cycle, significantly reducing false positives in critical monitoring applications like the Bio-Shield project.
The systematic design and simulation of these four distinct genetic circuits (including the copy of the repressilator) have provided critical insights into the fundamental principles of synthetic biology:
- Oscillatory Dynamics: The Repressilator validated the necessity of balanced negative feedback loops for emergent periodic behavior, demonstrating the system’s sensitivity to kinetic parameters.
- Metabolic Load and Stability: The analysis of the J23100 promoter variant confirmed that transcriptional imbalance disrupts functionality, proving that design modularity requires precise component tuning.
- Logical Processing: The NOT gate (Inverter) successfully implemented Boolean logic, providing a robust framework for modular signal processing.
- Environmental Sensing: The Threshold Biosensor integrated the previous designs into a functional system where reporter expression is cooperatively gated by environmental inputs. Together, these experiments confirm that robust biological design relies on the convergence of temporal control, metabolic balance, logical modularity, and threshold-based sensitivity. These findings establish a solid foundation for the Bio-Shield project, as they provide the strategies required to implement low-noise, logically-gated, and temporally-regulated biosensors suitable for real-world environmental safety applications.