E. coli cells are more likely to incorporate foreign DNA if their cell walls are altered so that DNA can pass through more easily. Such cells are said to be "competent." Cells are made competent by a process that uses calcium chloride and heat shock. Cells that are undergoing very rapid growth are made competent more easily than cells in other stages of growth.
The growth rate of a bacterial culture is not constant. In the early hours (lag phase), growth is very slow because the starting number of dividing cells is small. This is followed by a time of rapid cell division known as the log phase. The actual length of each phase depends on the temperature at which the cells are incubated. In this lab, you will start with cells that should be in the log phase.