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I am aware of the fact that $CaCl_2$ settles down on the cell wall making it less negative may be by forming bond with Teichoic acid. Also due to the positive charge it attracts DNA (DNA is negatively charged due to phosphate group). But is there anything else the $CaCl_2$ does? Also if its the work of $CaCl_2$ why we are adding $MgCl_2$ first in the standard protocol of the making competent cell preparation chemically?

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    $\begingroup$ Even after so many years, nobody really knows what CaCl₂ does. $\endgroup$ – WYSIWYG Aug 10 '16 at 17:55
  • $\begingroup$ @WYSIWYG That's really true. Exact mechanism is not really known. Everybody hypothesis that this may be the reason. But what exactly happen maybe few people knows. $\endgroup$ – Nil Aug 11 '16 at 19:28
  • $\begingroup$ Please run a spelling check before posting a question. Whatever your first language, you can set it to English. We expect posters to be "comeptent" enough to do that. $\endgroup$ – David Apr 24 '17 at 18:17
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Although we do not have any conclusive proof about how $CaCl_2$ works, the suggested mechanism is much similar to what you have suggested. When $CaCl_2$ dissociates in aqeous solution, $Ca^{2+}$ ions bind to the phosphate groups in DNA as well as the inner core of lipopolysaccharides in cell membrane (Dagert et al, 1979), neutralizing them. After this, heat shock allows uptake of this DNA into the host (how heat shock works is also not known; it is suggested that increased temperature creates pores in cell membrane, see this answer). For visual representation, see this diagram (from here):

calcium chloride transformation

Regarding $MgCl_2$, the role of $Mg^{2+}$ is not completely understood either. However, $Mg^{2+}$ is known to promote cell growth (Hanahan et al, 1983). It might be because $Mg^{2+}$ is an important factor required for functioning of ribosomes (Nierhaus et al, 2014). Anyhow, its role in increasing competence seems to be lesser than that of $Ca^{2+}$.

P.S.: during the experiment, $Mn^{2+}$, $Rb^+$ and $K^+$ also work effectively along with $Ca^{2+}$ (Hanahan et al, 1983).

References:

  1. Dagert, M.; Ehrlich, S. (1979). "Prolonged incubation in calcium chloride improves the competence of Escherichia coli cells". Gene. 6 (1): 23–28. doi:10.1016/0378-1119(79)90082-9. PMID 383576

  2. Douglas Hanahan, Studies on transformation of Escherichia coli with plasmids, Journal of Molecular Biology, Volume 166, Issue 4, 1983, Pages 557-580, ISSN 0022-2836, http://dx.doi.org/10.1016/S0022-2836(83)80284-8.

  3. Nierhaus K.H. $Mg^{2+}$, $K^+$, and the ribosome J. Bacteriol. 2014 196 3817-3819

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