Competent Host (For Transformation with Recombinant DNA)

A recombinant DNA molecule cannot simply float free once it has been assembled - it needs to be propagated inside a living host cell, and several kinds of host are available: E. coli, yeast, or animal/plant cells, with the actual choice depending on what the specific cloning experiment requires. E. coli remains the default, most widely used choice, for a combination of practical reasons: its genetics are extremely well characterised, it is easy and cheap to grow, it can accept a wide range of different vector types, its safety profile has been extensively studied, and under good growth conditions it divides roughly every 20 minutes - an extremely fast doubling time that lets a researcher amplify a recombinant molecule to usable quantities very quickly. Getting the DNA into the cell in the first place is not trivial, though, because DNA is a hydrophilic molecule and simply cannot cross a normal, intact bacterial cell membrane on its own. To get around this, bacterial cells are first deliberately made 'competent' - typically by treating them with a specific concentration of a divalent cation such as calcium - after which recombinant DNA can actually be forced into the cell by incubating the treated cells with the DNA on ice, briefly heat-shocking them at 42°C, and then returning them to ice; this cold-hot-cold cycle is what actually drives DNA uptake. For situations where the desired end product is a properly functioning eukaryotic protein rather than j …