Week 6 HW
DNA Assembly Homework
Phusion High-Fidelity PCR Master Mix contains a high-fidelity DNA polymerase, dNTPs, MgCl2, buffer salts, and stabilizers. The polymerase copies the DNA template, while its proofreading activity helps reduce mutations. The dNTPs are the building blocks used to make the new DNA strand. MgCl2 is needed for polymerase activity, and the buffer keeps the pH and salt conditions suitable for PCR. The master mix format also makes the reaction more consistent because many components are already premixed.
Primer annealing temperature depends mostly on the melting temperature of the primers. This is affected by primer length, GC content, sequence composition, and how well the primer matches the template. Longer primers and primers with higher GC content usually have higher melting temperatures. Salt and Mg2+ concentration in the reaction can also affect annealing. A good annealing temperature is usually a few degrees below the primer Tm, so the primers bind specifically but still efficiently.
PCR and restriction digests both make linear DNA fragments, but they do it in different ways. PCR uses primers and a DNA polymerase to amplify a chosen DNA region, so it is useful when you want to create many copies of a specific sequence or add designed overlaps for Gibson Assembly. Restriction enzyme digestion uses enzymes that cut DNA at specific recognition sites, so it is useful when the needed cut sites already exist in the plasmid or insert. PCR is more flexible because primers can be designed almost anywhere, but it can introduce mutations and requires good primer design. Restriction digestion is often simpler and reliable, but only works if the right enzyme sites are present and do not cut in unwanted places. For Gibson Assembly, PCR is often preferred when you need custom overlaps, while restriction digestion is convenient for opening a vector at known sites.
To make sure the digested and PCR-amplified DNA fragments are appropriate for Gibson cloning, the fragments should have matching overlaps, usually around 20 to 40 base pairs, at the ends that need to join. The overlaps should be unique, in the correct order and orientation, and should not have strong secondary structure or extreme GC content. I would also check the full planned assembly sequence in software such as Benchling to confirm that the junctions are correct. After PCR or digestion, I would verify fragment sizes using gel electrophoresis and clean up the DNA before assembly. It is also important to use the right molar ratios of vector and insert.
During bacterial transformation, plasmid DNA enters E. coli cells after the cells are made competent. In chemical transformation, calcium chloride and cold incubation help the DNA associate with the bacterial cell surface. A short heat shock then temporarily makes the membrane more permeable, allowing some plasmid DNA to enter the cells. After recovery in rich media, cells that took up the plasmid can grow on antibiotic plates if the plasmid contains the matching resistance gene. In electroporation, an electric pulse is used instead of heat shock to create temporary pores in the membrane.
Another DNA assembly method is Golden Gate Assembly. Golden Gate uses Type IIS restriction enzymes, such as BsaI or BsmBI, which cut outside of their recognition sequence. This lets the user design custom overhangs on each DNA part. In one reaction, the enzyme cuts the DNA pieces to create compatible sticky ends, and DNA ligase joins the pieces together. Because the recognition sites can be designed to disappear after assembly, the final product is not repeatedly cut again. This makes Golden Gate useful for assembling multiple parts in a defined order, such as promoter, coding sequence, and terminator modules. Compared with Gibson Assembly, Golden Gate depends more on designed restriction sites and overhangs, while Gibson depends on longer homologous overlaps. A simple diagram would show parts A, B, and C with unique sticky ends, then ligation into one circular plasmid in the correct order.