Mitosis, Maintaining Genetic Stability in Somatic Cells
On 2 February 1951, George Gey at Johns Hopkins University placed cervical cancer cells from a patient named Henrietta Lacks into culture. Those cells, now called HeLa, continued dividing and by 70+ years later had undergone an estimated 60,000+ mitotic divisions in laboratories worldwide. HeLa cells contain 70–80 chromosomes (a normal human somatic cell has 46), contributed to the development of the Salk polio vaccine in 1955, and became the first human cells used in space research in 1960. The HeLa line is living evidence that mitosis, when unregulated, can repeat indefinitely.
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
When George Gey placed Henrietta Lacks' cervical cancer cells in culture at Johns Hopkins in 1951, those cells began dividing, and they did not stop. Normal human somatic cells have built-in limits on how many times they can divide. HeLa cells lost that control. After 70+ years of continuous mitotic division they now carry 70–80 chromosomes instead of the normal 46, because errors in chromosome distribution accumulate when the normal checkpoints that govern cell division are overridden. Understanding why HeLa cells behave this way requires understanding how normal mitosis is supposed to distribute chromosomes accurately.
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.
DNA replicates BEFORE mitosis, producing identical sister chromatids joined together. This ensures there is a full copy of each chromosome to distribute to each daughter cell. Without prior replication, daughter cells would inherit incomplete hereditary information.
Pause, copy the highlighted pre-mitosis principle into your book before moving on.
The stages of mitosis: prophase, metaphase, anaphase and telophase.
Before mitosis can begin, the cell must first _____ its DNA.
Mitotic stages · line up, separate, redistribute
We just saw that DNA replication before mitosis provides identical sister chromatids, ready for distribution to daughter cells. That raises a question: how does the cell actually sort and separate those chromatids? This card answers it → the four stages of mitosis: prophase, metaphase, anaphase, telophase.
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.
Prophase: chromosomes condense, nucleus breaks down. Metaphase: chromosomes align at equator. Anaphase: sister chromatids separate to opposite poles. Telophase + cytokinesis: new nuclei form, cytoplasm divides → two daughter cells with same chromosome number as parent cell.
Add the highlighted stage sequence to your notes before the check below.
In which stage of mitosis do chromosomes line up at the cell equator?
Chromosome number · stability not reduction
We just saw that mitosis has four stages that line up and separate sister chromatids into two new nuclei. That raises a question: what is the actual result for chromosome number, does it change? This card answers it → mitosis maintains chromosome number, giving daughter cells the same count as the parent.
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. |
Mitosis maintains chromosome number: daughter cells have the same chromosome count as the parent somatic cell. Daughter cells are genetically very similar to the cells they replace, supporting continuity within the organism. Mitosis does NOT routinely create variation.
Pause, write the highlighted stability principle into your book.
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
We just saw that mitosis maintains chromosome number in daughter cells, supporting genetic stability within the organism. That raises a question: if mitosis is so useful, why does another type of division exist? This card answers it → contrasting mitosis (stability, somatic) with meiosis (reduction, gametes) and the key contexts where each is used.
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.
Mitosis functions: growth, repair, some asexual reproduction, all situations requiring genetically stable copies. Meiosis (coming next) reduces chromosome number for gamete formation and supports sexual reproduction. They have different purposes and different outcomes.
Add the highlighted functions and the mitosis-vs-meiosis contrast to your notes before the check below.
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
Pick your answer, then rate your confidence, that tells the system what to drill next.
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."
Show all answers
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).
George Gey's 1951 HeLa cell culture, derived from Henrietta Lacks' cervical cancer at Johns Hopkins, illustrates both what mitosis is supposed to do and what happens when its regulation fails. Normal mitosis ensures that after DNA replication each sister chromatid is pulled to opposite poles so each daughter cell receives the same chromosome complement as the parent cell. HeLa cells have accumulated to 70–80 chromosomes (versus the normal 46) because uncontrolled division means errors in chromatid separation go uncorrected, the very failures that normal mitotic checkpoints are designed to prevent. The cell line has now undergone 60,000+ divisions over 70+ years, contributing to the Salk polio vaccine (1955) and 11,000+ published papers, making it one of the most consequential biological preparations in history.