Competent Cells for Molecular Cloning and Recombinant Expression

Chemically competent cells use heat shock and require minimal equipment, while electrocompetent cells use an electrical pulse and often achieve higher efficiency. Electroporation is more sensitive to salts in the DNA preparation.

Possible causes include inactive cells, incorrect antibiotic, poor DNA, failed ligation, insufficient recovery or incompatible plasmid and strain. Test the cells with a control plasmid to separate transformation failure from construct failure.

Moderate efficiency is often enough for a purified plasmid, while ligations, assemblies and libraries benefit from higher efficiency because fewer correct molecules may be present.

Conductive salts increase current during the pulse and can produce an arc that kills cells. Desalt DNA or use a smaller volume of a clean preparation.

Some can express proteins, but dedicated expression strains often provide better control, polymerase systems, protease reduction or folding support. Choose a strain matched to the expression vector.

Repetitive, toxic or unstable sequences can recombine or be selected against. Use a stability-optimized strain, lower growth temperature and minimize culture duration where appropriate.

Keep them at the specified frozen temperature, usually in an ultralow freezer, and avoid thaw-refreeze cycles. Transport and bench handling should preserve the cold chain.

Use a known supercoiled control plasmid to verify cell competency and antibiotic selection. A no-DNA control helps identify contamination or incorrect selection.