Give the step-by-step reasoning process and then the final answer. Under standard growth conditions, the bacterial strain, E.coli, has a doubling time of 20 minutes.  If 1 single bacterial cell is used to start a culture grown under standard growth conditions, how many bacterial cells will there be after the culture is grown for 4 hours?
4 hours is 4 * 60 = 240 minutes. If the doubling time is 20 minutes, then after 240 minutes, the number of bacteria would have doubled 240 / 20 = 12 times. Thus, starting with only 1 bacterial cell, after 4 hours, there will be 1 * 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 = 4,096 bacterial cells.
The final answer: 4096.