E. Coli pGLO Transformation Lab?

In the transformation protocol, CaCl is often added to the mixture of DNA and bacteria. CaCl provides calcium cation (positively charged ion). The cation is added to neutralize the negative charged phosphate backbone of the DNA molecule and also the negatively charged surface of the bacteria. Thus, after you add the DNA, you have to let it sit for 30 minutes. The 30 minutes incubation on ice allows the bacterial membrane to stabilize and to increase the interaction between the calcium cation and the negatively charged components. Another theory is that calcium coats the bacteria surface, so that the negative charged DNA can be attached to the phospholipid membrane.

After that, the DNA-bacteria mixture is put into the 42C or 37C water bath for heat shock. The heat shock changes the fluidity of the membrane. Once the fluidity changes, DNA can then enter the bacteria at an efficient rate perhaps by cell surface invagination.

After heat shock for 30 seconds, the mixture is immediately placed on ice for 1-2 minutes before adding culture medium for growing. The exact reason for on-ice incubation is still under speculation. It is thought that the 2 minutes on-ice incubation reduces the thermal motion of DNA, so that it promotes exogenous DNA to enter the bacteria.