AQA GCSE Biology (8461) — Cell Biology
Under perfect conditions, a single bacterium could theoretically multiply to cover the entire Earth in just a few days. This explosive multiplication happens through a simple process called binary fission.
The process happens in four strict steps:
Important note: Every time a bacterium divides, each daughter cell receives exactly one copy of the main circular DNA, but they can receive a variable number of plasmids.
How do we predict how many bacteria will be in a population after several hours?
Bacterial populations experience exponential growth because each cell divides into two during every cycle. The time it takes for one cell to divide is called the mean division time. Under ideal conditions (optimum warmth, moisture, and plenty of nutrients), this can be as fast as 20 minutes for some bacteria.
To calculate the final population size, use these two steps:
(Note: represents the number of divisions. Ensure both times are in the exact same units before dividing!)
Because bacteria multiply so quickly, the numbers get too large to write out normally. For Higher Tier exams, you must be able to express your final answer in standard form (, where is between 1 and 10).
A culture starts with 5 bacteria. The mean division time is 30 minutes. How many bacteria will be present after 8 hours? Give your answer in standard form.
Step 1: Convert units so they match.
Step 2: Calculate the number of divisions ().
Step 3: Calculate the final population.
Step 4: Convert to standard form.
Rounding rule: AQA exams frequently ask for answers to a specific number of significant figures (e.g., 2 or 3 sig figs). If this question asked for 3 significant figures, the final answer would be . Always keep the full number in your calculator until this final step!
Students often describe binary fission using terms like 'nucleus', 'chromosomes', or 'mitosis' — bacteria do not have these, so always use the terms 'circular DNA' and 'prokaryotic division' instead.
When asked to 'describe' binary fission in a multi-mark question, examiners explicitly look for the phrases 'circular DNA moves to opposite poles' and the formation of 'new cell walls'.
When calculating population size, students frequently forget to multiply by the initial number of bacteria if the starting population is greater than 1.
The formula for final population size is not provided on the AQA equation sheet, so you must memorise it for the exam.
In calculation questions, always ensure your total time and mean division time are in the same units (usually minutes) before dividing to find the number of divisions.
Higher Tier students must remember that failing to convert the final answer into standard form usually results in the loss of the final accuracy mark, even if the core calculation is correct.
Binary fission
A type of simple cell division (asexual reproduction) used by prokaryotic cells to reproduce, where the cell replicates its genetic material and splits into two.
Prokaryotic cells
Simple cells, such as bacteria, that do not contain a true nucleus or membrane-bound organelles.
Daughter cells
The two new, genetically identical cells produced when a parent cell divides.
Circular DNA
The main loop of genetic material found in prokaryotic cells that is not enclosed in a nucleus.
Plasmids
Small, circular loops of DNA separate from the main bacterial chromosome that can replicate independently.
Opposite poles
The two far ends of a cell where the genetic material migrates before cell division.
New cell walls
The structures that form down the middle of a dividing bacterial cell to separate the two new daughter cells.
Exponential growth
A pattern of growth where the population size doubles during every division cycle.
Mean division time
The average amount of time it takes for one bacterial cell to divide into two.
Standard form
A mathematical notation used for writing very large or very small numbers using powers of 10, written as A x 10^n.
Put your knowledge into practice — try past paper questions for Biology
Binary fission
A type of simple cell division (asexual reproduction) used by prokaryotic cells to reproduce, where the cell replicates its genetic material and splits into two.
Prokaryotic cells
Simple cells, such as bacteria, that do not contain a true nucleus or membrane-bound organelles.
Daughter cells
The two new, genetically identical cells produced when a parent cell divides.
Circular DNA
The main loop of genetic material found in prokaryotic cells that is not enclosed in a nucleus.
Plasmids
Small, circular loops of DNA separate from the main bacterial chromosome that can replicate independently.
Opposite poles
The two far ends of a cell where the genetic material migrates before cell division.
New cell walls
The structures that form down the middle of a dividing bacterial cell to separate the two new daughter cells.
Exponential growth
A pattern of growth where the population size doubles during every division cycle.
Mean division time
The average amount of time it takes for one bacterial cell to divide into two.
Standard form
A mathematical notation used for writing very large or very small numbers using powers of 10, written as A x 10^n.
