The heat shock method is the standard protocol for artificial bacterial transformation (uptake of recombinant plasmid DNA). Competent E. coli cells prepared with divalent cations (such as $CaCl_2$) on ice are briefly subjected to a sudden thermal pulse at $42^\circ\text{C}$ for 45–90 seconds followed by immediate chilling on ice. This rapid temperature shift creates a transient thermal imbalance that increases membrane permeability and opens dynamic pores through which negatively charged DNA molecules enter the cell.