Mitosis — Maintaining Genetic Stability in Somatic Cells
When you heal a cut or grow from childhood to adulthood, cells are dividing constantly. Those new cells must usually keep the same chromosome number and the same core hereditary information as the original body cells. Mitosis is the process that makes that possible.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
If skin cells divide to repair a wound, the new cells must still function as skin cells. That means they must keep the correct chromosome number and the same core instructions as the original tissue.
Before reading on, predict what a cell division process would need to do to maintain stability. What would go wrong if the chromosomes were not shared evenly between daughter cells?
Know
- That DNA is replicated before mitosis begins.
- The main stages of mitosis at HSC depth.
- That chromosome number is maintained in daughter cells.
Understand
- Why mitosis supports growth and tissue repair.
- Why mitosis contributes to asexual reproduction in some organisms.
- How mitosis differs in purpose from meiosis without conflating the two.
Can Do
- Describe the stages of mitosis in order.
- Explain how chromosome number remains constant.
- Link mitosis to continuity within an organism.
Core Content
Before mitosis · DNA replication first
Mitosis does not start from unprepared DNA. Accurate sharing depends on DNA replication happening first.
Before mitosis, the cell replicates its DNA. Each chromosome is therefore copied, producing two identical sister chromatids joined together. This does not change the number of chromosomes yet in the way students usually count them at HSC level; it means each chromosome now exists in a replicated form ready for separation.
This preparation matters because when the cell divides, each daughter cell must receive one complete set of chromosomes. Without prior DNA replication, daughter cells would not inherit the full hereditary information needed for normal cell function.
The stages of mitosis: prophase, metaphase, anaphase and telophase.
What to write in your book
- DNA replicates BEFORE mitosis → each chromosome = two identical sister chromatids.
- Replication ensures there is a full copy to distribute to each daughter cell.
- Each daughter cell must receive one complete set of chromosomes.
- Without prior replication, daughter cells would lack full hereditary information.
Before mitosis can begin, the cell must first _____ its DNA.
Mitotic stages · line up, separate, redistribute
The stages of mitosis are best understood as a sequence that lines up, separates and redistributes identical chromatids.
| Stage | What happens |
|---|---|
| Prophase | Chromosomes condense and become visible. The nucleus begins to break down. |
| Metaphase | Chromosomes line up at the cell equator, ready for separation. |
| Anaphase | Sister chromatids separate and move to opposite poles. |
| Telophase | New nuclei form around each chromosome set. |
| Cytokinesis | The cytoplasm divides, producing two daughter cells. |
At HSC level, the core logic is simple: replicated chromosomes are organised, separated and distributed so that each new nucleus receives one complete set. The result is two daughter cells with the same chromosome number as the original somatic cell.
What to write in your book
- Prophase: chromosomes condense, nucleus breaks down.
- Metaphase: chromosomes line up at the equator.
- Anaphase: sister chromatids separate to opposite poles.
- Telophase + cytokinesis: new nuclei form, cytoplasm divides → two daughter cells.
In which stage of mitosis do chromosomes line up at the cell equator?
Chromosome number · stability not reduction
Mitosis is about stability, not reduction. The daughter cells keep the same chromosome number as the parent somatic cell.
If a parent body cell has a given chromosome number, each daughter cell produced by mitosis has that same number after division. This is why mitosis supports continuity within the organism: cells formed for growth or repair are genetically very similar to the cells they replace.
| Stage | What happens to hereditary material | Why it matters |
|---|---|---|
| Before mitosis | DNA replicates so each chromosome has two identical chromatids. | Ensures there is a full copy to distribute to each daughter cell. |
| During mitosis | Chromatids are separated to opposite poles. | Allows each future cell to receive one complete set. |
| After cytokinesis | Two daughter cells exist with the same chromosome number as the parent cell. | Maintains genetic stability in body tissues. |
What to write in your book
- Mitosis maintains chromosome number: parent number = daughter number.
- Daughter cells are genetically very similar to the cells they replace.
- This supports continuity within the organism.
- Mitosis does not routinely create variation (that's meiosis).
Mitosis halves the chromosome number in the daughter cells.
Mitosis produces two genetically identical diploid daughter cells.
Crossing over between homologous chromosomes occurs during prophase of mitosis.
Function and contrast · preview of meiosis
Mitosis is essential for growth because multicellular organisms need more cells as body size increases. It is also central to repair, replacing damaged or dead cells with new cells of the same type. In some organisms, mitosis also supports asexual reproduction because new individuals can be produced by repeated stable cell division.
This lesson also sets up a crucial contrast with meiosis. Mitosis maintains chromosome number in somatic cells, while meiosis will later be shown to reduce chromosome number for gamete formation. The two processes therefore have different purposes and outcomes.
Mitosis
- Occurs in somatic cells.
- Maintains chromosome number.
- Supports growth, repair and some asexual reproduction.
Preview of meiosis
- Occurs in formation of gametes.
- Reduces chromosome number.
- Supports sexual reproduction rather than tissue maintenance.
What to write in your book
- Mitosis: growth, repair, some asexual reproduction.
- Mitosis maintains chromosome number in somatic cells.
- Meiosis (next lesson): reduces chromosome number for gametes.
- Different processes, different purposes and outcomes.
Mitosis is mainly used for which of the following?
Activities
Order and Reason
For each event, state where it belongs in the sequence and explain why it is needed.
Event A. Sister chromatids move to opposite poles.
Event B. DNA replicates before cell division.
Event C. Chromosomes line up at the equator.
Event D. Two daughter cells form after cytokinesis.
Apply Mitosis to Context
Explain how mitosis is important in each case.
1. Healing a cut in the skin.
2. Growth of a child into an adult.
3. Asexual reproduction in a simple organism.
Core idea
- Mitosis maintains genetic stability by producing daughter somatic cells with the same chromosome number as the parent cell.
Mechanism / process
- DNA replicates first, chromosomes line up and separate during mitosis, and cytokinesis forms two daughter cells.
Common mistake
- Confusing mitosis with meiosis and claiming chromosome number is reduced.
Exam sentence starter
- "Mitosis supports growth and repair because it produces..."
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
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UnderstandBand 3(3 marks) 1. Outline the main stages of mitosis and the role of cytokinesis.
AnalyseBand 4(4 marks) 2. Explain how mitosis maintains chromosome number in daughter cells.
EvaluateBand 5–6(5 marks) 3. Evaluate the statement: "Mitosis is more important for growth and repair than for creating variation."
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Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Activity 1 — Order and Reason
Correct order: B (DNA replicates before cell division) → C (chromosomes line up at the equator) → A (sister chromatids move to opposite poles) → D (two daughter cells form after cytokinesis).
Why this matters: Replication provides a full copy, alignment prepares even separation, anaphase separates chromatids, and cytokinesis completes formation of two cells.
Activity 2 — Apply Mitosis to Context
1. Skin repair depends on mitosis to replace damaged somatic cells with genetically similar new cells.
2. Growth requires repeated mitosis so body cell number increases while chromosome number is maintained.
3. In asexual reproduction, mitosis can generate new individuals by repeated stable cell division.
Short Answer Model Responses
Q1 (3 marks): In prophase, chromosomes condense and the nucleus begins to break down [1]. In metaphase, chromosomes line up at the equator, and in anaphase they separate to opposite poles before telophase reforms nuclei [1]. Cytokinesis then divides the cytoplasm to produce two daughter cells [1].
Q2 (4 marks): Before mitosis, DNA replication produces duplicated chromosomes made of identical sister chromatids [1]. During mitosis, chromosomes line up and the sister chromatids are separated to opposite poles [1]. New nuclei then form around each chromosome set [1]. After cytokinesis, each daughter cell has the same chromosome number as the original somatic cell, so chromosome number is maintained [1].
Q3 (5 marks): The statement is correct because mitosis mainly supports growth and repair rather than creating variation [1]. Mitosis produces daughter somatic cells with the same chromosome number as the parent cell [1]. This makes it suitable for tissue growth, wound healing and routine replacement of body cells [1]. Its main role is maintaining genetic stability, not reducing chromosome number or generating new combinations of alleles [1]. Therefore, mitosis is more important for stable cell replacement and organism maintenance than for creating variation [1].
Before mitosis
DNA replicates so each chromosome is ready to be shared accurately.
Main outcome
Two daughter somatic cells with the same chromosome number as the parent cell.
Main functions
Growth, repair and some asexual reproduction.
Exam trap
Do not say mitosis halves chromosome number or mainly creates variation.
Rapid-fire questions on the stages of mitosis, maintaining chromosome number, and growth and repair. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
You should now be able to explain that stable body tissues depend on mitosis because replicated chromosomes are shared evenly into daughter cells. If chromosomes were not distributed accurately, growth and repair would not maintain normal cell function.