Identify bacterial binary fission in a sequence
Follow one bacterial cell through copying and separation.
Section B15.1 · Asexual reproduction
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall cells, DNA, inherited characteristics and the distinction between growth of an organism and production of offspring.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
After one bacterium completes one binary fission event, how many daughter cells are present?
What you will learn
- Read the events in the correct order
- Distinguish bacterial fission from mitosis
Identify bacterial binary fission in a sequence
| Feature | Meaning |
|---|---|
| Source | One bacterial cell |
| Copying | Parental DNA copied before separation |
| Outcome | Two daughters, not three cells |
| Cell organisation | Bacteria have no membrane-bound nucleus |
Read the events in the correct order
In bacterial binary fission, one bacterial cell copies its DNA, grows and partitions into two daughter cells. The original cell is not retained as an additional third cell after separation. In the supplied sequence, each daughter receives a copy of the parental genetic information, so the process is an example of asexual reproduction in the ordinary no-mutation model.
The diagram shows stages rather than several independent parents. Read from one original cell, through DNA copying and constriction, to two separated cells. Two DNA loops drawn within one dividing cell do not mean that two gametes have fused. A diagram key may use loops or colours to track copying; those are model symbols, not a measurement of every bacterial chromosome shape.
Distinguish bacterial fission from mitosis
Bacteria have no membrane-bound nucleus and do not undergo the nuclear division process called mitosis. Their binary fission must not be labelled as bacterial mitosis. Some eukaryotic single-celled organisms also divide into two, but that does not make every fission mechanism identical. Here the organism is explicitly a bacterium, so use the appropriate cell organisation.
Identifying fission in a written model does not require culturing bacteria. Use the supplied sequence and counts. If a photograph contains two nearby cells but does not show a connecting division stage or reproductive history, proximity alone is weaker evidence. Look for the copying-and-separation information rather than assigning a mechanism from any pair of shapes.
Sources for this lesson
Pause and explain
Why is bacterial fission not called mitosis?
Worked example
A bacterial cell copies its DNA and separates into two cells. How many cells exist after the event, and why is it asexual?
Show the worked solution
- The parent cell becomes two daughters, not two daughters plus the old cell.
- One parent supplies copied information without gamete fusion.
Answer Two daughter cells; the supplied one-parent copying process is asexual binary fission.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Put these events in order: separation, DNA copying, constriction of the cell.
Give me a hint
Copy information before partition and separation.
Compare my reasoning
- DNA is copied.
- The cell constricts and then separates.
DNA copying → constriction → separation into two daughter cells.
Look for these in your work
- Order copying before separation
- End with two daughter cells
Why does the model with two DNA copies in one dividing bacterium not show fertilisation?
Give me a hint
Identify where the copies came from.
Compare my reasoning
- The DNA copies came from one cell.
- There is no fusion of gamete nuclei.
The DNA copies are produced within one bacterial cell; they are not nuclei from two fusing gametes.
Look for these in your work
- Trace the one-cell source
- Reject gamete-fusion inference
A caption calls bacterial division mitosis because two daughters appear. Correct it.
Give me a hint
Two daughters alone do not establish a nuclear division process.
Compare my reasoning
- Bacteria lack a membrane-bound nucleus.
- Their division is binary fission, not bacterial mitosis.
The process is bacterial binary fission. Bacteria lack a membrane-bound nucleus and do not undergo mitosis.
Look for these in your work
- Name binary fission
- Explain the nuclear-organisation distinction
Common mistakes
- Counting the original cell again after it has divided.
- Calling bacterial binary fission mitosis.
What can you explain now?
Why is bacterial fission not called mitosis?
Compare with the explanation
Bacteria do not divide a membrane-bound nucleus
Their cell organisation differs from eukaryotic nuclear division.
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Make it stick
Draw and label a one-to-two bacterial fission sequence and explain why the original cell is not a third cell.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.