Explain why normal mitosis maintains the chromosome number
Normal mitosis maintains the chromosome number in each daughter. Six before copying leads to six in each completed daughter, not six split between them.
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
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
A six-chromosome parent completes normal mitosis. How many chromosomes are in each daughter?
What you will learn
- Trace a full complement into each daughter
- Distinguish maintained number from reduction
Original teaching model. Counts and records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Parent comparison | Six chromosomes before replication |
| Daughter 1 | Six chromosomes |
| Daughter 2 | Six chromosomes |
| Both daughters combined | Twelve chromosomes across two cells |
Trace a full complement into each daughter
Normal mitosis maintains the chromosome number in each daughter nucleus. The cell first copies its chromosome DNA, then distributes one copy of every chromosome to each product. The copying makes it possible to produce two complete complements rather than splitting a single uncopied list into two incomplete halves. This is the key connection between preparation and the result.
In a stipulated cell with six chromosomes before copying, each daughter cell after normal mitosis and subsequent cell division has six chromosomes. There are twelve chromosomes when the contents of both daughters are added together, but six in each individual daughter. The total across a population and the number per nucleus are different quantities.
Distinguish maintained number from reduction
A division that changes the stated diploid complement to a haploid complement does not fit the chromosome-maintaining mitotic outcome. Meiosis is treated separately in the next syllabus section; this contrast is only a boundary for identifying mitosis, not a replacement for the full meiosis sequence. Gamete formation and fertilisation must not be inserted into the ordinary mitotic copying model.
Count the supplied model at comparable points: the parent before replication and the completed daughter nuclei after separation. During preparation, each replicated chromosome has two attached DNA copies, which can create a counting trap. The later lessons explicitly distinguish chromosome number from DNA-copy number so the maintained-number explanation does not silently change its counting rule.
Pause and explain
Which comparison correctly tests maintained number?
Worked example
A parent has six chromosomes. What number is expected in each normal mitotic daughter?
Show the worked solution
- Use maintained number per daughter.
- Separate that from the sum across both products.
Answer Each daughter has six; both daughters together contain twelve.
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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.
State how copying and separation maintain a six-chromosome complement.
Give me a hint
Give each daughter every represented chromosome.
Compare my reasoning
- Copy each chromosome before mitosis.
- Separate the copies into two full complements.
Each chromosome is copied before mitosis, then one copy of each is distributed to each daughter nucleus, giving six in each.
Look for these in your work
- Include copying before division
- Explain complete distribution
Two completed daughters each contain eight chromosomes. State the per-cell number and their combined total.
Give me a hint
Do not confuse one cell with both.
Compare my reasoning
- Read eight in each cell.
- Add eight plus eight.
The per-cell number is eight; the combined total across the two cells is sixteen.
Look for these in your work
- State the per-cell value
- Label the combined total
A record stipulates that a diploid six-chromosome parent gives haploid three-chromosome products. Does it describe normal mitosis?
Give me a hint
Compare the number and chromosome sets.
Compare my reasoning
- Identify the reduction from six to three.
- Compare with the maintained-number definition.
No. The stated reduction from diploid to haploid is inconsistent with normal chromosome-maintaining mitosis; full meiosis teaching remains a separate section.
Look for these in your work
- Identify the reduction
- Keep the wider section boundary
Common mistakes
- Use per-cell counts when explaining maintained chromosome number.
- Mitosis is not the diploid-to-haploid reduction division.
What can you explain now?
Which comparison correctly tests maintained number?
Compare with the explanation
Parent before replication versus each completed daughter nucleus
Use comparable complete complements.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, trace a full complement into each daughter and distinguish maintained number from reduction. 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.