分子克隆

Molecular cloning requires an insert—the DNA fragment you want to copy, which can come from prokaryotes, eukaryotes, or be artificially created—and a vector, typically plasmid DNA that replicates in host cells. The vector carries recognition sites for restriction enzymes and contains an origin of replication and antibiotic resistance gene for selection.

Plasmids contain an antibiotic resistance gene that allows bacteria carrying the plasmid to survive on antibiotic-containing media. When transformed bacteria are plated on agar with antibiotics and incubated overnight at 37°C, only colonies with the plasmid survive, making antibiotic selection an effective screening method for successful transformation.

Gel purification separates and isolates desired DNA fragments after restriction enzyme digestion. For plasmids, this step removes uncut circular plasmid that appears as high molecular weight smear, leaving only linearized plasmid ready for ligation with the insert. This ensures clean, pure DNA for downstream steps.

Planning ensures you select restriction sites absent from your insert to avoid cutting it during digestion. You must also verify that your chosen sites exist in the plasmid's multiple cloning site. This strategy prevents failed cloning attempts and saves time by identifying potential issues like needing blunt-end ligation before beginning.

A 3-to-1 insert-to-vector ratio is recommended for DNA ligation reactions. This ratio ensures minimal self-ligation of the vector alone. After setup on ice, the ligation is incubated between 14-25°C for 1 hour to overnight to allow DNA ligase to join the insert and plasmid.

Sequencing serves as the final verification step to confirm that the gene of interest has been correctly cloned into the plasmid. After restriction digestion confirms insert presence on a gel, bacteria are expanded and plasmids purified. Sequencing then verifies the insert sequence matches the desired DNA fragment.

Molecular cloning enables creation of cDNA libraries from mRNA, reconstruction of biosynthetic pathways by reorganizing gene clusters between bacterial strains, generation of mutant libraries using error-prone polymerases, and incorporation of reporter genes like green fluorescent protein into plasmids to visualize gene expression and viral infection.