Week 6 HW: hw-genetic-circuits-part-i
Week 6 — Genetic Circuits I: DNA Assembly Technologies
Molecular Biology Lab Report: PCR & Assembly Techniques
Components of Phusion High-Fidelity PCR Master MixPhusion Master Mix is a convenient 2X concentrated solution containing:Phusion DNA Polymerase: A pyrococcus-like enzyme fused with a processivity-enhancing domain. It provides extremely high fidelity ($50\times$ higher than Taq) and speed.dNTPs: The building blocks ($dATP, dTTP, dCTP, dGTP$) for the new DNA strand.Reaction Buffer: Maintains optimal pH and provides ionic strength.MgCl2: A necessary cofactor for polymerase activity.
Factors Determining Primer Annealing Temperature ($T_m$)The optimal annealing temperature ($T_a$) is typically $3–5^\circ C$ below the $T_m$ of the primers. $T_m$ is determined by:Base Composition: The ratio of G-C pairs (3 hydrogen bonds) to A-T pairs (2 hydrogen bonds). Higher GC content increases $T_m$.Primer Length: Longer primers generally have higher melting temperatures.Salt Concentration: Monovalent cations ($Na+$) and divalent cations ($Mg{2+}$) stabilize the DNA duplex, raising $T_m$.Mismatches: Any base pair mismatch significantly lowers the stability and $T_m$.
PCR vs. Restriction Enzyme Digests Both methods generate linear DNA fragments, but they differ significantly in application and protocol.
| Feature | PCR (Polymerase Chain Reaction) | Restriction Enzyme Digest |
|---|---|---|
| Mechanism | De novo synthesis using a DNA polymerase and specific primers. | Physical “cutting” of existing DNA at specific recognition sequences. |
| Output | Exponentially amplified copies of a specific target region. | Fragments produced from a limited amount of template (no gain in mass). |
| Speed/Efficiency | Highly efficient; can create billions of copies from a tiny sample. | Limited by the amount of starting material; throughput depends on substrate mass. |
| Specificity | High; defined by custom primer sequence design. | Fixed; defined by natural recognition sites (e.g., GAATTC for EcoRI). |
Key Differences Summary
- Amplification vs. Fragmentation: PCR is a constructive process that increases the total amount of DNA. Restriction digestion is a reconstructive/analytical process that breaks down existing DNA into smaller pieces.
- Flexibility: PCR allows researchers to target almost any sequence by designing new primers. Restriction digestion is constrained by the presence of specific enzyme motifs (palindromes) within the DNA sequence.
- Sensitivity: PCR can detect and amplify DNA from single cells or degraded samples, whereas Restriction Enzyme Digests usually require microgram quantities of high-quality DNA for visualization on a gel.