Check the usual human chromosome count through normal mitosis
Usual human model: forty-six chromosomes before copying, forty-six replicated chromosomes with ninety-two DNA molecules while sisters are attached, and forty-six chromosomes in each completed daughter.
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall chromosomes, genes, chromosome sets, selective gene expression and the distinction between a nucleus and a whole cell.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many chromosomes are in each completed daughter in the usual human diploid mitotic model?
What you will learn
- Maintain forty-six chromosomes per daughter
- Separate attached DNA copies from per-cell totals
Original teaching model. Counts and records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Before copying | 46 chromosomes; 23 pairs |
| Attached-copy stage | 46 replicated chromosomes; 92 DNA molecules |
| Each completed daughter | 46 chromosomes; 23 pairs; two sets |
| Both daughters combined | 92 chromosomes across two cells |
Maintain forty-six chromosomes per daughter
The usual human diploid model starts with forty-six individual chromosomes, arranged as twenty-three pairs. Before normal mitosis, the chromosome DNA is replicated. At the attached-copy stage there are forty-six replicated chromosomes with ninety-two DNA molecules. Copying has supplied the information for two complete daughter complements; it has not turned the cell into ninety-two separate cells.
When the sister copies separate and the daughter nuclei form, each receives forty-six chromosomes. After the simple completed cell-division outcome, each daughter has twenty-three pairs and two sets. Twenty-three individual chromosomes is the usual human gamete model from a different division pathway, not the expected per-daughter outcome of ordinary diploid mitosis.
Separate attached DNA copies from per-cell totals
Adding both daughter complements gives ninety-two individual chromosomes across two cells. That combined quantity does not mean either cell has ninety-two chromosomes. Compare the parent before replication with each completed daughter, and label every intermediate count. If asked about the entire undivided cell after sister separation, the counting context differs from the endpoint per-nucleus question.
This is the usual human count model used by the syllabus, not a universal statement about every human cell or a clinical chromosome test. Mature red blood cells lack a nucleus, and biological chromosome-number variations exist. The teaching diagram supplies numbers rather than a diagnostic karyotype. It helps explain normal chromosome maintenance without predicting an individual's health or condition.
Pause and explain
At the attached-copy stage, forty-six replicated chromosomes contain how many DNA molecules?
Worked example
What are the normal completed daughter counts for a usual forty-six-chromosome diploid parent?
Show the worked solution
- Maintain the parent complement in each product.
- Keep the combined total separate.
Answer Each daughter has forty-six chromosomes; both together contain ninety-two.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
Trace the usual human count from the parent before replication to each completed daughter.
Give me a hint
Compare the endpoints per nucleus.
Compare my reasoning
- Start with forty-six individual chromosomes.
- Give the same full complement to each daughter.
The parent before replication has forty-six chromosomes; each completed mitotic daughter also has forty-six, arranged as twenty-three pairs in the usual model.
Look for these in your work
- State both endpoint counts
- Maintain the diploid model
Calculate the attached-stage DNA-molecule count and the combined completed-daughter chromosome total.
Give me a hint
Label two different ninety-two values.
Compare my reasoning
- Calculate forty-six times two DNA molecules.
- Add forty-six chromosomes in each of two daughters.
There are ninety-two DNA molecules at the attached-copy stage, and ninety-two chromosomes across the two completed daughters; each daughter contains forty-six.
Look for these in your work
- Distinguish molecules and chromosomes
- State the per-daughter limit
A report says ordinary mitosis turns a usual human diploid cell into two twenty-three-chromosome cells. Repair it.
Give me a hint
Mitosis maintains, rather than halves, the complement.
Compare my reasoning
- Identify the incorrect reduction.
- Replace it with forty-six in each daughter.
Normal mitosis gives two daughter cells with forty-six chromosomes each in this usual diploid model; two twenty-three-chromosome products would describe a reduction, not this outcome.
Look for these in your work
- Repair both products
- Retain the usual-model qualification
Common mistakes
- Ninety-two attached DNA copies are not ninety-two completed daughter chromosomes in one nucleus.
- Twenty-three per product is not normal diploid mitosis.
What can you explain now?
At the attached-copy stage, forty-six replicated chromosomes contain how many DNA molecules?
Compare with the explanation
Ninety-two
Each replicated chromosome has two DNA molecules.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, maintain forty-six chromosomes per daughter and separate attached dna copies from per-cell totals. Distinguish nuclear division, chromosome copies and completed daughter cells before opening the solutions.
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.