transformation of competent bacteria
You have built your recombinant plasmid in a test tube. Now you need a living factory to copy it — but bacterial cells do not normally let loose DNA in through their wall. Transformation is the step where bacteria take up DNA from their surroundings, and 'competent' bacteria are cells specially treated so they will accept it.
Most bacteria do not naturally absorb DNA well, so the lab makes them competent — receptive to DNA. The classic chemical method bathes E. coli in cold calcium chloride, which helps the negatively charged DNA stick to the cell surface; a brief 42 degrees Celsius heat shock then jolts some DNA across the membrane. An alternative, electroporation, applies a quick high-voltage pulse that punches transient pores in the membrane for DNA to slip through. Either way, only a small fraction of cells actually take up a plasmid, but because a single transformed cell will divide into millions of identical descendants overnight, even a low success rate gives you plenty of cells carrying your construct.
Transformation is the bridge from test-tube chemistry to living biology — the moment your engineered DNA enters a cell that can replicate it. After transformation you spread the cells on a selective plate so that the selectable marker on the plasmid lets only the transformed cells grow into colonies. Each colony is a clone: a pure population descended from one original cell, all carrying identical copies of your plasmid.
Mix your ligation product with ice-cold competent E. coli, hold on ice, plunge to 42 degrees Celsius for 45 seconds, back on ice, let the cells recover in rich broth for an hour, then spread on an antibiotic plate. Next morning each colony is a tower of millions of cells, all carrying copies of your plasmid.
Heat shock pushes plasmid DNA into chemically competent E. coli.
'Transformation' here means bacterial DNA uptake — not to be confused with the same word's other meaning in cancer biology, where a 'transformed' cell is one that has turned cancerous.