Distinguish meiosis mitosis and fertilisation using the supplied nuclear evidence
Compare the event, starting complement and per-nucleus outcome together.
Section B16.2 · Cell division
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-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.
Before you begin
- Recall chromosomes, genes and alleles, complete chromosome sets, mitosis and fusion of gamete nuclei.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which event is fusion rather than division?
What you will learn
- Compare reduction maintenance and fusion
- Evaluate what chromosome counts can and cannot establish
Original teaching model. Counts, source combinations and sample records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Normal diploid mitosis | 6 → 6 in each daughter; maintained complement |
| Normal meiotic reduction | 6 → 3 in each final product; one complete set |
| Normal fertilisation | 3 + 3 → 6 in the zygote nucleus |
| Evidence limit | Six alone does not establish which event occurred |
Compare reduction maintenance and fusion
Use the process and the supplied starting state together. In a normal diploid mitosis model, chromosome copies separate so daughter nuclei retain the precursor complement and genetic information. In meiosis, the final product nuclei have one complete set rather than two, and genetic combinations can differ. In fertilisation, two gamete nuclei fuse and their contributions form the zygote nucleus.
For a stipulated species with n equal to three, normal diploid mitosis gives two completed daughter cells with six chromosomes in each nucleus. A complete normal meiotic count model gives four haploid product nuclei with three chromosomes each. Fusion of two normal gamete nuclei gives a zygote nucleus with six. These endpoint numbers help apply an explicitly identified process; they are not an exhaustive rule for all possible cell events.
Evaluate what chromosome counts can and cannot establish
Six chromosomes in a single nucleus does not by itself prove that mitosis has just occurred. It could be a diploid precursor, a mitotic daughter, or a zygote in the stated model. Likewise, two observed cells do not prove normal mitosis without their history and nuclear evidence. Nuclear count, chromosome identity, cell boundaries and the described event each supply different parts of the explanation.
Do not equate genetic appearance with demonstrated sequence identity. The normal mitosis explanation describes matching information after accurate copying and separation; meiosis explains changed combinations and reduction; fertilisation combines contributions. When a question supplies only a count, state the interpretation that the model supports and name the missing event or genetic evidence instead of claiming certainty about the entire process.
Sources for this lesson
Pause and explain
A diploid precursor with six chromosomes gives normal final products with three per nucleus in this model. Which process fits the stated reduction?
Worked example
In a stipulated n = 3 model, compare 6 → 6 per daughter, 6 → 3 per final product and 3 + 3 → 6.
Show the worked solution
- Maintaining six per daughter matches normal diploid mitosis.
- Reducing six to three matches meiosis.
- Joining two three-chromosome nuclear contributions matches fertilisation.
Answer Process history plus the reference complement identifies maintenance, reduction or fusion; the number six alone cannot identify the event.
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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.
Classify these stipulated events: A retains six per daughter nucleus after division; B reduces six to three per final product; C joins two three-chromosome nuclei to give six.
Give me a hint
Match maintenance, reduction and fusion.
Compare my reasoning
- Match A with maintained complement.
- Match B with halved complement.
- Match C with fusion.
A fits normal diploid mitosis, B meiosis, and C fertilisation in the supplied n = 3 model.
Look for these in your work
- Correct A and B
- Correct C with fusion
You observe one nucleus with six chromosomes in an n = 3 species. Does that prove mitosis has just occurred? Give two other compatible states.
Give me a hint
A count is an endpoint, not a complete event history.
Compare my reasoning
- Identify six as the diploid complement.
- Consider a precursor and a newly formed zygote.
- State the missing process evidence.
No. A diploid precursor or a zygote formed by normal gamete fusion can also have six. Event history and appropriate nuclear evidence are needed to identify recent mitosis.
Look for these in your work
- Rejects count-only proof
- Gives precursor and zygote alternatives
An image shows two cells of similar appearance but no chromosome records or event history. Evaluate a claim that they are genetically identical mitotic daughters.
Give me a hint
Separate appearance, cell count and genetic evidence.
Compare my reasoning
- Identify the limited observed features.
- Request event history or nuclear/genetic records.
- Avoid a sequence-identity conclusion from appearance.
The claim is not established. Similar appearance and two cell boundaries do not prove their origin or complete genetic identity; division history and appropriate nuclear or genetic evidence are needed.
Look for these in your work
- Limits appearance-based inference
- Requests relevant event or genetic evidence
Common mistakes
- Using a count of six as proof of one particular event
- Confusing nuclear division with fusion
- Ignoring the starting ploidy and reference complement
What can you explain now?
A diploid precursor with six chromosomes gives normal final products with three per nucleus in this model. Which process fits the stated reduction?
Compare with the explanation
Meiosis
Two complete sets become one complete set.
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Make it stick
Without your notes, compare reduction maintenance and fusion and evaluate what chromosome counts can and cannot establish. State the chromosome number per nucleus and distinguish chromosome-set reduction from fertilisation.
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.