Final answer:
Competent cells must be kept on ice to stabilize their membranes for DNA uptake during transformation and to ensure the integrity and viability of cells for cryopreservation and transportation.
Step-by-step explanation:
It is important to keep competent cells on ice at all times because the cold temperature is essential for maintaining cell membrane integrity and keeping the metabolism of the cells low. This is critical when preparing cells for transformation, such as with E. coli cells treated with calcium chloride. The cold helps to stabilize the cells, allowing them to more readily take up foreign DNA when the temperature is later shifted (transformation). If competent cells are not kept on ice, their membranes may change (crystallize), adversely affecting their ability to take up DNA. In addition, cold temperatures are necessary for cryopreservation to ensure long-term viability of cell lines or clinical specimens that may need to be stored or transported.