| MadSci Network: Cell Biology |
Dear questionner,
Thanks for your interesting questions. Believe it or not, transformation of bacteria using DNA remains a rather mysterious process. Often in science we find a procedure that works, but we don't always understand fully how it works - this is an "empirical finding", meaning found through experiment. Furthermore, since artificial transformation is a practical, laboratory technique, many people don't really care how it works - they are just happy that it does!
I have searched PubMed, the major online database of bio-medical-related scientific literature, using terms such as "bacteria transformation plasmid heat shock mechanism" and obtained thi s set of 265 articles and 2 review articles. I would not expect you to look at all of them, but you might like to skim through and read the abstracts where the title looks relevant. One in particular caught my attention:
Panja S, Aich P, Jana B, Basu T. (2008)These authors found that heat-shock and the subsequent return of cells to 0 °C releases lipids and proteins from the outer membrane of the cell, making it less fluid. They also observed that the inner membrane became depolarised. Normally, the cells maintain what's called a membrane potential by pumping protons from inside - outside: the cell interior is therefore negatively-charged relative to the exterior. Depolarisation means that this membrane potential was lost.
How does plasmid DNA penetrate cell membranes in artificial transformation process of Escherichia coli?
Mol Membr Biol. 25(5):411-422
A simple interpretation of these observations is that heat-shock makes the cells leaky. Pores formed in the outer membrane permit DNA to enter and the loss of membrane potential helps the negatively-charged DNA to cross the inner membrane.
As to why 42 °C is the optimal temperature for heat-shock: this again is empirical. Since E. coli grows fastest at 37 °C, anything much lower than 42 °C would not induce heat-shock, anything much higher would be lethal to the cells.
Hope that helps with your question,
Neil
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