During which growth phase are cells actively dividing by binary fission, causing the population to increase exponentially?

• A. Lag phase
• B. Death phase
• C. Persister cells
• D. Log phase (exponential)

Answer: (D)

In the log phase, cells actively divide by binary fission, causing exponential population growth. After the initial adaptation period, nutrients are abundant and waste is manageable, so each cell splits into two at a fairly constant generation time. As a result, the total cell count rises rapidly and doubles at regular intervals, producing an exponential curve on a growth chart. This differs from the lag phase, where cells are mostly adapting and dividing slowly; the stationary phase, where growth slows and stops due to nutrient depletion and waste buildup; and the death phase, where more cells die than are produced. Persister cells are a subset that survive stress by becoming dormant, not a phase of rapid growth.