Competent bacteria are given thermal shock for forcing the entry of rDNA into host bacteria at the following temperature.

In rDNA technology, the host bacteria are made competent by treating them with CaCl2, which make the rDNA pass through the holes of cell wall and comes in contact with the cell membrane. Cell membrane is impermeable to DNA as DNA is negatively charged. The host bacterial cells are incubated with rDNA on ice or at low temperatures followed by placing them briefly at 42oC (heat shock) and then putting them back on ice. This makes the bacteria taking up the recombinant DNA.