Re: How do CaCl2 and heat shock work together to make cells competent?

Dear questionner,

Thanks for your question. I'm afraid to say: molecular biologists have been asking "what's the mechanism of calcium chloride-induced competence" ever since they discovered the technique. Do any literature search using terms such as "calcium chloride transformation bacteria DNA mechanism" and you're sure to find a sentence similar to "the mechanism of DNA uptake by calcium chloride-induced competent cells is not fully understood".

This seems rather unsatisfactory! However, the lack of understanding has not prevented development of many variations on the theme. Researchers have substituted other salts (such as rubidium chloride), altered the DNA incubation time, heat-shock duration and a host of other conditions. I think that this illustrates an important point: this is a practical technique and many researchers are concerned less with how it works and more with improving the method - mostly so as to save themselves some time.

What we do know is that the process is physicochemical, rather than physiological: in other words, it is different to the natural uptake of DNA from the environment, seen in some species of bacteria. We can speculate about the process using our knowledge of chemistry and physics. As you suggest, the phosphate backbone of DNA is negatively-charged, so it is likely that the calcium ions promote binding between DNA and the outer surface of the cell. Incubation on ice will reduce the thermal motion of molecules and so further promote the binding process. On heat shock, we hypothesise that the cell membrane alters in a way that allows DNA uptake. One possibility is that the cell surface invaginates - bulges inwards then reseals, carrying surface-bound DNA into the cell. Another is that the cell becomes somewhat "leaky" for a very brief time - not enough to lose the internal contents, but enough that DNA can enter.

There is way too much literature on the topic to cover here, so I will point you to some resources that will help with your report. Start first with Google Scholar and try combinations of words or phrases such as: bacteria, DNA, plasmid, competence, transormation, calcium chloride. Look in particular for review articles which summarise the topic. You can also try PubMed, a database of scientific literature, with a similar search.

Hope that helps with your query,
Neil