Explanation

DNA being a hydrophilic molecule can not pass through cell membranes. Therefore, the bacteria should be made competent to accept the DNA molecule In this case the cell is treated with specific concentration of a divalent cation such as calcium to increase pore size in cell wall The cells are incubated with recombinant DNA on ice, followed by placing them briefly at $42℃$ and them putting it back on ice. This is called heat shock treatment. The bacteria now takes up the recombinant DNA