Distinguish chromosome copying from reduction in a meiotic count model
Copying increases DNA amount; reduction changes the complete sets per product nucleus.
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 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
Does copying the DNA automatically change a diploid nucleus to haploid?
What you will learn
- Separate DNA copying from chromosome-set reduction
- Interpret four-product counts without functional-gamete promises
Original teaching model. Counts, source combinations and sample records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Before copying | 6 chromosomes represented by 6 DNA molecules |
| Attached copies | 6 replicated structures represented by 12 DNA molecules |
| Four final nuclei | 3 individual chromosomes in each haploid nucleus |
| Combined final count | 4 × 3 = 12 across four nuclei; not per nucleus |
Separate DNA copying from chromosome-set reduction
A chromosome can be copied before division, producing attached sister copies. In the supplied count convention, the attached structure is counted as one replicated chromosome until its copies separate. Copying the DNA does not itself remove a chromosome set or make a diploid nucleus haploid. The precursor still represents two corresponding sets; the amount of DNA and the number of sets answer different questions.
Consider a normal schematic with a diploid complement of six. Before copying it has six chromosomes represented by six DNA molecules. After copying it has six replicated chromosome structures represented by twelve DNA molecules. At the end of a complete meiotic model there are four haploid product nuclei, each containing three individual chromosomes. Detailed named stages are not required for this syllabus section.
Interpret four-product counts without functional-gamete promises
Across those four final nuclei, the combined count is twelve chromosomes. In the stipulated no-loss count model that matches the twelve copied DNA molecules distributed among the products. It does not mean the starting diploid nucleus contained twelve chromosomes under the attached-copy convention, and it does not mean each final product has twelve. Keep time point, boundary and counting convention explicit.
The model explains how copying and subsequent division can leave each product with one complete set. It is not a claim that every organism produces four equivalent functional gametes. The number of mature gametes and their later development depend on the supplied biological context. If a question asks only for reduction, state two sets to one and the halved per-nucleus complement instead of inventing unrequested stage names.
Sources for this lesson
Pause and explain
In the supplied n = 3 complete model, how many chromosomes are in each final haploid nucleus?
Worked example
Trace a stipulated n = 3 precursor through copying and the final product count.
Show the worked solution
- Before copying, six chromosomes are represented by six DNA molecules.
- Copying gives six attached replicated structures represented by twelve DNA molecules.
- Four final haploid nuclei contain three chromosomes each, twelve across four boundaries.
Answer DNA copying is preparation; two-set to one-set reduction concerns the final complement per nucleus.
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Test this topic →From guided practice to a new situation
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.
A diploid model has six unreplicated chromosomes. Each is copied with its two sister copies still attached. State the replicated-structure count and DNA-molecule count under the supplied convention.
Give me a hint
Attached sisters remain one replicated structure.
Compare my reasoning
- Retain six attached structures.
- Count two DNA molecules per structure.
Six replicated chromosome structures and twelve DNA molecules; the precursor remains diploid in this model.
Look for these in your work
- Six structures
- Twelve DNA molecules without claiming haploid status
A stipulated species has n = 4. A complete normal meiotic schematic ends with four haploid nuclei. State the precursor diploid count, each final count and the combined final count.
Give me a hint
Keep 2n, n and four separate nuclei apart.
Compare my reasoning
- Calculate 2 × 4 for the precursor.
- Use four per haploid nucleus.
- Add 4 × 4 only for the combined total.
Eight chromosomes in the diploid precursor, four in each final haploid nucleus and sixteen across the four final nuclei.
Look for these in your work
- Eight precursor and four per product
- Sixteen only across four nuclei
A student sees twelve DNA molecules after copying in the n = 3 model and concludes reduction has already occurred. Explain the error.
Give me a hint
Copying changes DNA amount; reduction changes complete sets per nucleus.
Compare my reasoning
- Identify the six attached replicated structures.
- Retain two complete sets at that time point.
- Reserve reduction for the later haploid products.
Twelve DNA molecules represent copies in six attached replicated structures. The nucleus still has two sets; the later products are haploid with three chromosomes each. DNA copying alone is not reduction.
Look for these in your work
- Distinguishes DNA molecules from attached structures
- Places reduction in the later one-set products
Common mistakes
- Calling DNA copying a reduction to haploid
- Counting each attached sister copy as a separate chromosome without stating a different convention
- Assuming four equivalent functional gametes from a count diagram
What can you explain now?
In the supplied n = 3 complete model, how many chromosomes are in each final haploid nucleus?
Compare with the explanation
Three
Each product receives one complete set of three types.
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Make it stick
Without your notes, separate dna copying from chromosome-set reduction and interpret four-product counts without functional-gamete promises. 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.