Final answer:
Cells are incubated at 4°C to make E. coli competent for DNA uptake by allowing DNA to attach to the cells' exterior before the heat shock step facilitates entry into the cells.
Step-by-step explanation:
The student asked why cells were incubated at 4°C after adding heat-killed bacteria into the cell plate. The reason for this is to render the E. coli cells competent for DNA uptake.
When bacterial cells are exposed to a chilled calcium chloride solution and then incubated at 4°C, the cell membranes become more permeable, allowing the DNA molecules to attach to the exterior of the cells.
Following this cold incubation, a heat shock process at 42°C for 2 minutes is applied to facilitate the entry of the DNA into the bacterial cells. However, since only a small fraction (0.01%) of the cells undergo transformation, it's necessary to use a selective medium to identify and grow only the transformed cells.