Bacteria multiply by simple cell division (binary fission) as often as once…

The idea

Bacteria reproduce asexually by binary fission: the single circular chromosome is copied, the cell elongates, and the cell divides into two genetically identical daughter cells. Under ideal conditions, with sufficient nutrients and a suitable temperature, some bacteria can complete this cycle about every 20 minutes, so numbers can double each generation. Starting from one cell, the population after n divisions is 2ⁿ; after 20 minutes there are 2 cells, after 40 minutes 4, after 1 hour 8, and after 3 hours 512. In practice growth slows as nutrients run out and waste products accumulate, so the rate is not maintained indefinitely. In the required practical, students culture bacteria on agar plates using aseptic technique, keeping plates sealed and incubated at a temperature no higher than 25 °C in school laboratories to reduce the risk of growing pathogens.

What earns the marks

• Binary fission is asexual reproduction in which one bacterial cell divides into two genetically identical daughter cells.
• The circular DNA is replicated and the cell divides, so population size doubles each generation under ideal conditions.
• A generation time of about 20 minutes applies only when nutrients are sufficient and the temperature is suitable.
• Population size after n divisions can be calculated as 2ⁿ, or by repeated doubling.
• Growth eventually slows as nutrients are used up and toxic waste products build up.
• Culturing bacteria requires aseptic technique, sterilised equipment and incubation at a safe school temperature, no higher than 25 °C.

Common mistakes and how to put them right

• Confusing binary fission with mitosis.

  Put it right: Bacteria have no nucleus, so binary fission is not mitosis even though both produce genetically identical cells.

• Assuming the 20-minute doubling continues forever.

  Put it right: Nutrients become limiting and waste accumulates, so the rate falls.

• Calculating population growth by multiplying by the number of divisions instead of using powers of two, which underestimates growth.

  Put it right: Use 2ⁿ or repeated doubling.

Exam tips

• Show the doubling sequence or use 2ⁿ, and state the time unit clearly in your answer.
• When asked about conditions, name both sufficient nutrients and a suitable temperature, and explain why each matters.
• In practical questions, mention aseptic technique and the 25 °C maximum incubation temperature for school cultures.