Explain exact chromosome replication before mitosis
Four unreplicated chromosomes become four replicated chromosomes with eight attached DNA copies before mitosis. Each completed daughter later receives four chromosomes.
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
When does exact chromosome replication occur relative to mitosis?
What you will learn
- Place chromosome copying before nuclear division
- Distinguish copied information from extra cells
Original teaching model. Counts and records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Before replication | 4 chromosomes; 4 DNA molecules |
| Attached-copy stage | 4 replicated chromosomes; 8 DNA molecules |
| Timing | Replication precedes mitosis |
| Limit | mRNA messages are not whole-chromosome replicas |
Place chromosome copying before nuclear division
Exact chromosome replication occurs before mitosis. The DNA information in each chromosome is copied so that the nuclear division can distribute a complete matching complement to each daughter. Preparation therefore supplies the copies before they are separated; it is not an event that first happens after the two daughter nuclei have already received incomplete chromosome lists.
In the original four-chromosome model, each unreplicated chromosome contains one DNA molecule. After replication, each consists of two attached sister copies, each with a DNA molecule carrying the same copied sequence. At that attached stage there are four replicated chromosomes and eight DNA molecules. The two copies share the same chromosome identity in the stipulated exact-copying model.
Distinguish copied information from extra cells
The counting rule matters. A replicated chromosome is counted as one chromosome while its sister copies remain joined; when they separate they become daughter chromosomes. Comparing the parent before replication with each completed daughter gives four chromosomes in each. A drawing of eight attached DNA copies does not show eight cells, nor does it mean the final daughter nucleus receives eight chromosomes.
Exact replication is the expected accurate-copying model, not a claim that mutations are impossible. DNA information is copied using cellular processes and materials; the earlier expression lesson's mRNA copy is a different type of information use. Making one mRNA molecule does not replicate a whole chromosome or prepare the complete chromosome complement for mitotic division.
Pause and explain
Four replicated chromosomes each have two attached DNA copies. What counts apply at that stage?
Worked example
Four chromosomes are copied before division. Count the attached-stage chromosomes and DNA molecules.
Show the worked solution
- Keep each joined sister pair as one chromosome.
- Count two DNA molecules per replicated chromosome.
Answer There are four replicated chromosomes and eight DNA molecules at the attached-copy stage.
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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.
Explain why replication must precede the mitotic separation.
Give me a hint
Each daughter needs the complete information.
Compare my reasoning
- Copy the DNA in every parent chromosome.
- Provide one matching copy for each daughter.
Replication before mitosis supplies two matching copies of every chromosome, allowing a complete complement to be distributed to each daughter nucleus.
Look for these in your work
- State the timing
- Explain the purpose
Independently count a model with four chromosomes before copying and four joined sister pairs after copying.
Give me a hint
Count chromosome structures and DNA molecules separately.
Compare my reasoning
- Keep four joined pairs as four replicated chromosomes.
- Calculate four times two DNA molecules.
Before copying: four chromosomes and four DNA molecules. At the attached-copy stage: four replicated chromosomes and eight DNA molecules.
Look for these in your work
- Use the stated counting point
- Distinguish chromosomes and molecules
A model shows three mRNA molecules made from one gene. Does that establish replication of all chromosomes before mitosis?
Give me a hint
Separate gene expression from whole-complement preparation.
Compare my reasoning
- Identify mRNA as an information message.
- State that whole-chromosome DNA-copying evidence is missing.
No. Three mRNA messages do not establish exact DNA replication of the whole chromosome complement before mitosis.
Look for these in your work
- Distinguish RNA and DNA copying
- State the missing evidence
Common mistakes
- Replication occurs before mitosis, not after distribution.
- Attached sister copies and completed daughter chromosomes use different counting points.
What can you explain now?
Four replicated chromosomes each have two attached DNA copies. What counts apply at that stage?
Compare with the explanation
Four chromosomes and eight DNA molecules
Each joined structure is one replicated chromosome.
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Make it stick
Without your notes, place chromosome copying before nuclear division and distinguish copied information from extra cells. 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.