Heat shock time should be determined for different strains of cell. For DH5alpha or JM109 use 30-45 sec. For BL21 use 120 sec. Deactivate ligase prior to transformation. Ligase may reduce transformation efficiency.
Transformation efficiency has a roughly inverse relationship with the size of plasmids.
2 different plasmids can be transformed at the same time, or one after another. For routine transformation whereby efficiency of transformation is of no import, some of the steps may be shorten or omitted. Detergents may be detrimental to the transformability of the competent cells, therefore the glassware used for making competent cells should not be washed with detergents. Polycarbonate flask may also be used instead of glass flask. DMSO can dissolve polystyrene, therefore use polypropylene tubes.