Plasmid cloning from insert to colony

Plasmid cloning is a chain of claims. You claim the insert has the right ends, that it joined the vector, that a bacterium took up that molecule, that the colony on the plate contains it, and that the sequence matches the map. Each claim needs a different check. Skipping one is how a laboratory archives the wrong plasmid under a confident name. This page walks the chain from insert to a colony worth keeping. The chemistry of the join itself is in restriction ligation and Gibson assembly. This is a research explainer, not a kit insert.

Enzymes, competent-cell classes and plates are sourcing questions for the molecular biology catalogue. Put the map and the verification plan in the quote request.

What has to be true before the tube is mixed

A plasmid is a circular DNA that a host can copy. The pieces you care about are the origin of replication, a selectable marker, and the insert in a stated orientation relative to any promoter you intend to use. A high-copy origin is convenient for a prep and a problem if the insert is toxic. Write that down before you choose the backbone.

The insert arrives as a PCR product, a restriction fragment, or a synthetic piece. PCR inserts carry the error rate of the polymerase you used. A proofreading enzyme lowers that rate and does not make it zero, so a clone still needs a sequence. Synthetic DNA needs the same sequence check against the file you ordered. NCBI GenBank is one public place a reference sequence may live. Your map should cite the accession or the file, not a memory of the gene name.

Ends decide the joining method. Compatible sticky ends, blunt ends, or overlapping flanks for an isothermal assembly are not interchangeable. If the plan says two different restriction sites, both enzymes have to have cut, and the fragment on the gel should be the size that double digest predicts. Purify fragments away from the enzymes and the small off-cuts that would compete in the ligation. A gel that still shows the uncut parent is not a purified insert.

Assembly, cells and plates

Follow the end-matching rules in the companion note, and keep a ligase cold rather than leaving it on a warm bench. A vector-only control shows how much self-ligation to expect. A no-ligase control shows whether uncut vector is still in the mix. Those two tubes explain empty plates and carpets of empty vector.

Competent cells are a reagent with a measured performance: colonies per amount of a control plasmid, under stated conditions. Chemical competence and electroporation are different methods with different salt tolerances. A ligation full of salt, or a Gibson reaction used undiluted into electroporation, can fail in the cuvette rather than in the join. Transform a known plasmid in parallel. If that control yields no colonies, the new clone is not the thing to debug first.

Selection only works if the agar contains the antibiotic the marker gene resists, at a concentration appropriate to that marker and that medium. Those concentrations are plasmid-specific. Do not reuse a number from a different backbone. Ampicillin plates grow satellite colonies around true transformants because the enzyme that destroys ampicillin leaks into the agar. Satellites are small, appear late, and do not carry the plasmid reliably. Do not pick them, and do not incubate ampicillin plates until the lawn of satellites looks like a result. Other antibiotics have their own failure clocks. Read the marker you have.

Screening without fooling yourself

Pick colonies from a plate that is not a lawn. Restreak or inoculate several. Colony PCR across the insert, or a miniprep followed by a diagnostic digest, sorts empty vector from plasmids that gained DNA. Run the products on a gel next to a ladder, as in agarose gel electrophoresis for DNA. A band of the right size is still compatible with the wrong orientation if your primers did not ask about direction. Design at least one check that would fail if the insert were backwards, or sequence through both junctions.

Blue-white screening, on vectors that carry the lacZ fragment used for alpha complementation, makes interrupted plasmids more often pale or white when the indicator is in the agar. Colour is not a sequence. A frameshift in the lacZ piece without your insert can look white. A small in-frame insert can look blue. Use colour to choose which colonies to screen, then screen them.

Sequence the winner. Sanger reads across the insert and the junctions catch polymerase errors and chewed ends. A Sanger sequencing enquiry reference is a way to frame that check. It is an independent method reference, not a statement that a sequencing bench is already running in your name. Compare the read to the map base by base. "It looks about right" is how a missing codon enters a year of experiments.

When the plate disagrees with the plan

No colonies, and the control plasmid also fails: cells, plates, heat shock or the electroporator, not the insert. No colonies, and the control plasmid works: the join, the ends, a toxic insert, or salt in an electroporation. Colonies on the vector-only plate as well as the real plate: uncut or self-ligating vector. Screen more colonies or cut the vector more completely and, where the method allows, remove its 5-prime phosphates so it cannot close without an insert that brings phosphates.

Colonies that all lack the insert: the insert never competed. Check molar ratio class, cleanup, and whether the ends were compatible. One beautiful colony with a mutation in the middle of a PCR insert: that is polymerase history. Sequence a second colony before you redesign the primers. Mixed peaks in a "pure" clone often mean two plasmids in one pick. Restreak to single colonies and sequence again.

Safety, heat and the record

Recombinant DNA stays inside the institutional biosafety decision for the host and the insert. A cloning strain is still a living organism. Antibiotic resistance genes do not belong in a drain. Autoclave or otherwise treat waste as your committee requires. The ATCC culture guides are a public reference for how culture collections describe strain handling. They do not replace your local rule. This page is not an approval to clone a hazardous insert.

Agar plates in a hot, dry room lose water and crack, and colonies stay tiny. In humid weather they sweat, droplets spread cells between colonies, and a pick becomes a mixture. Dry plates just enough for the cells to stay where you plated them, and do not leave them open under a fan that blows spores across the bench. A power cut to an incubator means the selection time on the clock is fictional. Look at the colonies, do not assume six hours of heat happened because six hours passed.

Name the archived strain with the plasmid, the date, and the sequence file. A tube labelled with a gene nickname is how the wrong origin gets used next year. Freeze only the colony you sequenced, or a culture descended from it without another transformation.

What to put in an enquiry

EVRINTH can take a sourcing question. State the vector features you need, the insert length, whether the insert is a PCR product or a synthetic sequence, the host species, and how you will verify. If the insert should be synthesised rather than amplified, the custom gene synthesis enquiry reference is a prompt for that specification. Ask whether a quotation is possible. Do not describe only the gene name and expect the backbone, the codon usage and the junctions to be obvious.