Comparing chemical and electrocompetent cells

The plate in the hero photograph, blue and white colonies on a screening dish, is where both competence methods hope to arrive. It does not tell you which method can get a given DNA there. Chemical competence is a salt-and-heat-shock class. Electrocompetence is a washed, low-salt cell and a brief electrical pulse. The decision is which class matches the DNA you actually have, and which mistakes each class punishes. The shared culture steps, the recovery, and the antibiotic plate are in bacterial culture and transformation. How an insert becomes a candidate colony after that plate exists is plasmid cloning from insert to colony.

Cell and reagent classes can be named from the reagents and chemicals catalogue. The surrounding path is the molecular biology pathway. If the construct is still a design, custom gene synthesis is an enquiry reference, not a statement that DNA is already being made. A specification for the cell class goes through the quote request.

Two ways of making a membrane admit a plasmid

A working picture of chemical competence, the one the method is built on, is that cold cells in a divalent-cation buffer, often a calcium chloride class, sometimes with other salts or with additives the manufacturer lists, have their envelope prepared so a short heat shock lets plasmid DNA in. The heat shock used with many commercial preparations is a brief step near 42 Celsius. The time and the exact temperature belong to the preparation in your freezer. Copying a number from a paper that used a different preparation is how viability collapses. After the shock the cells go back on ice, then into rich broth with no antibiotic, for long enough that the resistance protein can be made before the drug is encountered.

Electrocompetence starts from the opposite ionic condition. Cells are washed in cold water or a very low-salt buffer until the suspending medium barely conducts. They are mixed with DNA that is itself in water, placed between the electrodes of a cuvette, and given a short high-voltage pulse. The field is what the method uses to drive DNA across the envelope. Immediate recovery medium then dilutes and feeds the cells. Settings, including the voltage class for a given cuvette gap, belong to the instrument and the cell sheet. This explainer will not print a pulse table to copy.

Efficiency is a lot property, usually discussed as colonies from a stated mass of a standard supercoiled plasmid. Electrocompetent lots are often chosen when that class must be high: large plasmids, which transform less readily, and library ligations, where the molecule you want is rare. A routine retransformation of a pure plasmid often succeeds with chemical cells and a simpler setup. The lot sheet wins every argument about the number. Public shapes of both workflows are collected in the Addgene bacterial transformation protocol and among Promega protocols.

What each tube is allowed to contain

Chemical cells expect salt. Their buffer is salty. A small volume of a typical ligation can be added, and many ligations are transformed this way without a cleanup, up to the volume where the ligation chemistry starts to inhibit colony formation. The manufacturer's ceiling on DNA volume is the ceiling. More DNA is not more kindness.

Electrocompetent cells are ruined by the ions a ligation buffer was designed to provide. Salt lowers resistance. The pulse, instead of a controlled field, becomes an arc: a pop, a flash, a dead time constant on instruments that report one, and cells that do not recover. Tris-EDTA storage buffer is already saltier than these cells want. A "few microlitres" of an uncleaned ligation can be enough. Polymers used in some assembly mixes do not rescue the ionic problem. The cleanup that matches this method is DNA in water or in a buffer the cell sheet names as low conductivity, at a volume the sheet allows.

Air bubbles in the cuvette gap and a cuvette that is warm or wet on the outside are mechanical ways to spoil a pulse that the DNA itself did not spoil. Reused cuvettes that were not cleaned and dried as the manufacturer allows carry the salts of the previous sample. Chemical tubes have their own mechanical failures: a heat block that was never at temperature, a water bath that was, and a timer that became a conversation.

The Addgene molecular biology reference is a public place to see ligations and transformations discussed as classes. Method repositories such as protocols.io show how laboratories record their own sheets. The sheet shipped with the cells remains the one you run.

When the control plasmid and the new DNA disagree

Plate a known intact plasmid beside the new DNA, and plate cells with no DNA, on the same antibiotic batch. Read the pair as a method check, not as a hunt through every upstream enzyme.

If the known plasmid and the new DNA both fail, and the no-DNA plate is clear, the cells, the heat block, the cuvette or the recovery are the common cause. An arc on the electroporator is a finding you can write down. A chemical tube that sat thawed in a rack while plates were labelled is a finding too. Thaw on ice, in a deep pan, and start the protocol clock.

If the known plasmid in water gives colonies and the ligation arcs or gives none, the salt and the volume are the difference. Clean the ligation or switch that sample to chemical cells that the salt does not forbid. If the known plasmid fails only when you add a mock ligation buffer, you have demonstrated the ionic problem without spending the real DNA.

If the no-DNA plate grows, stop comparing competence methods. The plates are not selective, and neither cell class can be judged. That branch belongs to the antibiotic and the pour, not to the pulse.

Recovery is shared and non-negotiable in both methods. Plating immediately onto drug, before the resistance protein exists, produces an empty plate from cells that did take up DNA. The recovery time is the one the cell sheet names. Rich broth, no antibiotic, then a measured plating.

Method, DNA, and the thing that ruins it

Handling that is shared, and handling that is not

Both classes are frozen reagents. Thaw them on ice and use them the day the sheet allows. Refreezing a thawed aliquot is a different, usually worse, reagent. Both need a recovery without antibiotic. Both are judged with a no-DNA plate and a known plasmid. Both fail if the antibiotic does not match the marker.

They part company on the DNA cleanup and on the physical tool. Chemical work needs a heat source you have checked that morning. Electroporation needs cuvettes of the gap the programme expects, a pulser someone has maintained, and DNA whose ionic strength you can defend. A library ligation cleaned into water is a different sample from the same ligation scooped from its buffer. Write which one you have on the tube before you book the machine.

Detergent left in a "clean" tube lyses either kind of cell. Autoclaved water that was actually tris buffer will arc. Labels are part of the method.

Research-use limits

These are non-pathogenic cloning strains under the institutional assessment you already have. Nothing here is a method for introducing DNA into a pathogen or into a strain the committee has not seen. Voltages and heat are hazards to the operator as well as variables in a protocol. Follow the instrument training. This page is not medical advice and not a pulse programme.

Ice in a hot room

A shallow ice pan in a room that is already hot becomes water while you label plates. Chemical cells lose competence as they warm. Electrocompetent cells, low in salt and high in density, are often the first to die in that wait. Keep the ice deep, the tubes buried, and the cuvettes cold for the interval the sheet allows. A shipment that arrives thawed is not the lot whose efficiency was printed. In an enquiry, state frozen storage, the shipping temperature class, and that electroporation DNA will be supplied in water if that is the method. A warm courier handoff should be visible in the packaging specification, not discovered when the arc is the first result.

What the enquiry should name

Name the strain genotype, chemical or electrocompetent, the storage class, and whether you need the higher efficiency class because the plasmid is large or the ligation is a library. Ask for the lot specification of efficiency and for the DNA buffer the cells expect. The method can be discussed against that sheet. Send the note through the quote request. Upstream assembly choices that decide how salty the DNA will be can be lined up with the molecular biology pathway, and any sequence that still has to be designed stays on the custom gene synthesis reference as a question, not as a run in progress.