B2: Cell Division
The cell cycle, mitosis, meiosis, and stem cells
Key Definitions
Chromosome — A coiled structure of DNA containing genetic information; found in the nucleus of eukaryotic cells.
Gene — A short section of DNA that codes for a specific protein.
Mitosis — A type of cell division that produces two genetically identical daughter cells with the same number of chromosomes as the parent cell.
Meiosis — A type of cell division that produces four genetically different gametes, each with half the number of chromosomes of the parent cell.
Stem cell — An undifferentiated cell that can divide repeatedly and differentiate into specialised cell types.
Differentiation — The process by which a cell becomes specialised for a particular function.
Haploid — A cell containing one set of chromosomes (e.g. gametes: 23 in humans).
Diploid — A cell containing two sets of chromosomes (e.g. body cells: 46 in humans).
Gene — A short section of DNA that codes for a specific protein.
Mitosis — A type of cell division that produces two genetically identical daughter cells with the same number of chromosomes as the parent cell.
Meiosis — A type of cell division that produces four genetically different gametes, each with half the number of chromosomes of the parent cell.
Stem cell — An undifferentiated cell that can divide repeatedly and differentiate into specialised cell types.
Differentiation — The process by which a cell becomes specialised for a particular function.
Haploid — A cell containing one set of chromosomes (e.g. gametes: 23 in humans).
Diploid — A cell containing two sets of chromosomes (e.g. body cells: 46 in humans).
The Cell Cycle
The cell cycle has three main stages:
1. Growth (G1 phase) — The cell grows, makes more organelles (ribosomes, mitochondria), and synthesises proteins.
2. DNA Replication (S phase) — The cell's DNA is copied so that each new cell will receive a complete set. The number of chromosomes stays the same but each chromosome now consists of two identical sister chromatids joined at the centromere.
3. Mitosis (M phase) — The cell divides: the chromatids are pulled apart and two genetically identical daughter cells are produced, each with the same number of chromosomes as the parent cell.
1. Growth (G1 phase) — The cell grows, makes more organelles (ribosomes, mitochondria), and synthesises proteins.
2. DNA Replication (S phase) — The cell's DNA is copied so that each new cell will receive a complete set. The number of chromosomes stays the same but each chromosome now consists of two identical sister chromatids joined at the centromere.
3. Mitosis (M phase) — The cell divides: the chromatids are pulled apart and two genetically identical daughter cells are produced, each with the same number of chromosomes as the parent cell.
Exam tip: The cell cycle includes growth AND DNA replication BEFORE mitosis happens. Don't forget — DNA replication must occur first so each daughter cell gets a full copy of the genetic material.
Mitosis: Stages and Significance
Stages of mitosis:
1. Prophase — Chromosomes condense and become visible; the nuclear membrane begins to break down.
2. Metaphase — Chromosomes line up at the centre (equator) of the cell.
3. Anaphase — Sister chromatids are pulled apart to opposite poles by spindle fibres.
4. Telophase — Nuclear membranes reform around each set of chromosomes; the cytoplasm divides (cytokinesis).
Significance of mitosis:
— Growth — Produces new cells for multicellular organisms to grow.
— Repair — Replaces damaged or worn-out cells (e.g. skin cells, blood cells).
— Asexual reproduction — Some organisms reproduce by mitosis alone (e.g. bacteria, strawberry runners, potato tubers).
1. Prophase — Chromosomes condense and become visible; the nuclear membrane begins to break down.
2. Metaphase — Chromosomes line up at the centre (equator) of the cell.
3. Anaphase — Sister chromatids are pulled apart to opposite poles by spindle fibres.
4. Telophase — Nuclear membranes reform around each set of chromosomes; the cytoplasm divides (cytokinesis).
Significance of mitosis:
— Growth — Produces new cells for multicellular organisms to grow.
— Repair — Replaces damaged or worn-out cells (e.g. skin cells, blood cells).
— Asexual reproduction — Some organisms reproduce by mitosis alone (e.g. bacteria, strawberry runners, potato tubers).
Example 1: Why Mitosis Produces Identical Cells
Before mitosis, the DNA is replicated so each chromosome consists of two identical sister chromatids. During anaphase, the chromatids are separated — one goes to each new cell. Since the chromatids were identical copies, both daughter cells receive exactly the same DNA. This is why mitosis produces genetically identical daughter cells.
Example 2: Mitosis in Action — Skin Repair
When you cut your skin, cells at the wound edge divide by mitosis. Each cell division produces two identical daughter cells that are genetically the same as the original skin cell. These new cells fill in the gap and eventually differentiate into specialised skin cells, restoring the damaged tissue.
Meiosis
Meiosis is a type of cell division that produces gametes (sex cells):
— Involves two rounds of division (meiosis I and meiosis II).
— Produces four genetically different daughter cells (gametes).
— Each gamete has half the number of chromosomes (haploid) of the parent cell.
— In humans: a diploid cell (46 chromosomes) → 4 haploid gametes (23 chromosomes each).
Why meiosis is necessary: Gametes must be haploid so that when two gametes fuse during fertilisation, the resulting zygote has the correct diploid number of chromosomes (23 + 23 = 46 in humans).
Genetic variation in meiosis arises from:
1. Independent assortment — Chromosomes are distributed randomly between the gametes.
2. Crossing over — Homologous chromosomes exchange sections of DNA during meiosis I.
— Involves two rounds of division (meiosis I and meiosis II).
— Produces four genetically different daughter cells (gametes).
— Each gamete has half the number of chromosomes (haploid) of the parent cell.
— In humans: a diploid cell (46 chromosomes) → 4 haploid gametes (23 chromosomes each).
Why meiosis is necessary: Gametes must be haploid so that when two gametes fuse during fertilisation, the resulting zygote has the correct diploid number of chromosomes (23 + 23 = 46 in humans).
Genetic variation in meiosis arises from:
1. Independent assortment — Chromosomes are distributed randomly between the gametes.
2. Crossing over — Homologous chromosomes exchange sections of DNA during meiosis I.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two |
| Number of daughter cells | Two | Four |
| Chromosome number | Same as parent (diploid) | Half of parent (haploid) |
| Genetic variation | Daughter cells are genetically identical | Daughter cells are genetically different |
| Where it occurs | Body cells (somatic cells) | Gonads (testes and ovaries) |
| Purpose | Growth, repair, asexual reproduction | Production of gametes for sexual reproduction |
Example 3: Chromosome Number Through Meiosis and Fertilisation
A human body cell has 46 chromosomes (23 pairs). During meiosis in the testes/ovaries, the chromosome number is halved to 23 in each gamete (sperm or egg). When a sperm (23) fertilises an egg (23), the zygote has 46 chromosomes again — restoring the diploid number. This zygote then divides by mitosis to grow into an embryo and eventually a fully developed human.
Example 4: Why Gametes Must Be Haploid
If gametes were diploid (46 chromosomes each), fertilisation would produce a zygote with 92 chromosomes — double the normal number. This would be fatal. Meiosis ensures gametes are haploid (23 chromosomes) so that fertilisation restores the correct diploid number (46).
Stem Cells
Embryonic stem cells — Found in early embryos; totipotent (can differentiate into ANY type of cell in the body). Most flexible but raise ethical concerns.
Adult stem cells — Found in adult tissues (e.g. bone marrow, brain, skin); multipotent (can only differentiate into a limited range of cell types related to their tissue of origin). Less flexible than embryonic stem cells but fewer ethical issues.
Differentiation in animals — Happens mainly during early development. Most animal cells lose the ability to differentiate after becoming specialised (with the exception of adult stem cells).
Differentiation in plants — Plant cells retain the ability to differentiate throughout their entire life. Meristems (regions of unspecialised cells) are found at the tips of roots and shoots and can produce any type of plant cell.
Adult stem cells — Found in adult tissues (e.g. bone marrow, brain, skin); multipotent (can only differentiate into a limited range of cell types related to their tissue of origin). Less flexible than embryonic stem cells but fewer ethical issues.
Differentiation in animals — Happens mainly during early development. Most animal cells lose the ability to differentiate after becoming specialised (with the exception of adult stem cells).
Differentiation in plants — Plant cells retain the ability to differentiate throughout their entire life. Meristems (regions of unspecialised cells) are found at the tips of roots and shoots and can produce any type of plant cell.
Example 5: Bone Marrow Transplant
A patient with leukaemia (cancer of white blood cells) can be treated with a bone marrow transplant. The patient's own diseased bone marrow is destroyed using chemotherapy/radiotherapy. Healthy bone marrow from a donor (containing adult stem cells) is then transplanted. The donor stem cells differentiate into new, healthy blood cells, replacing the cancerous cells.
Therapeutic Cloning and Ethical Issues
Therapeutic cloning — An embryo is produced that is genetically identical to the patient. The embryo's stem cells are harvested and used to generate replacement tissues or organs. Because the cells are genetically identical to the patient, they will not be rejected by the immune system.
Ethical issues with stem cells:
— Embryonic stem cells require the destruction of an embryo, which some people believe is a potential human life.
— Embryos used are often leftover from IVF treatment — some argue they would be destroyed anyway.
— Adult stem cells avoid the embryo issue but are less versatile.
— Therapeutic cloning raises concerns about the creation and destruction of embryos for medical purposes.
— There are also concerns about the source of embryonic stem cells and informed consent.
Ethical issues with stem cells:
— Embryonic stem cells require the destruction of an embryo, which some people believe is a potential human life.
— Embryos used are often leftover from IVF treatment — some argue they would be destroyed anyway.
— Adult stem cells avoid the embryo issue but are less versatile.
— Therapeutic cloning raises concerns about the creation and destruction of embryos for medical purposes.
— There are also concerns about the source of embryonic stem cells and informed consent.
Example 6: Potential Uses of Stem Cells in Medicine
Stem cells have the potential to treat: Type 1 diabetes (replace insulin-producing cells in the pancreas), spinal cord injuries (replace damaged nerve cells), Parkinson's disease (replace dopamine-producing brain cells), heart disease (repair damaged cardiac muscle), and burns (grow new skin tissue). Research is ongoing and many treatments are still in the experimental stage.
Exam tip: When asked about ethical issues with stem cells, always present both sides. For example, some argue that using embryos is wrong because they have the potential for life, while others argue the potential medical benefits outweigh this concern and the embryos would otherwise be destroyed.
Practice Questions
1. Foundation Describe the three main stages of the cell cycle.
1) Growth — the cell grows and produces more organelles and proteins. 2) DNA replication — the cell's DNA is copied so each new cell receives a full set. 3) Mitosis — the cell divides into two genetically identical daughter cells.
2. Foundation State two differences between mitosis and meiosis.
1) Mitosis produces two daughter cells; meiosis produces four. 2) Mitosis produces genetically identical diploid cells; meiosis produces genetically different haploid cells. 3) Mitosis involves one division; meiosis involves two. (Any two.)
3. Higher A human body cell contains 46 chromosomes. How many chromosomes would be in: (a) a sperm cell, (b) a zygote, (c) a cell after the S phase of the cell cycle but before mitosis?
(a) 23 chromosomes (haploid gamete produced by meiosis). (b) 46 chromosomes (diploid zygote formed by fertilisation: 23 + 23). (c) 46 chromosomes, but each chromosome now consists of two sister chromatids — the DNA has been replicated but the chromosome count is still 46.
4. Higher Explain why meiosis is important for sexual reproduction.
Meiosis halves the chromosome number to produce haploid gametes. This ensures that when two gametes fuse at fertilisation, the diploid number is restored. Without meiosis, the chromosome number would double each generation. Meiosis also introduces genetic variation through crossing over and independent assortment, which is the basis of natural selection and evolution.
5. Foundation Discuss the ethical issues surrounding the use of embryonic stem cells.
Some people believe destroying embryos is wrong because they have the potential for human life and deserve the same rights. Others argue the embryos used are often leftover from IVF and would be destroyed anyway, and the potential to cure serious diseases outweighs the ethical concerns. Adult stem cells are an alternative but are less versatile than embryonic stem cells.
6. Higher Explain the advantage of therapeutic cloning over using donor stem cells.
In therapeutic cloning, the embryo is genetically identical to the patient, so the stem cells produced will have the same DNA. This means the replacement tissues or organs will not be rejected by the patient's immune system, unlike donor stem cells which may be recognised as foreign and attacked.
🔢 Maths Skills
Mathematical Skills for this Topic
Calculating cells after N mitotic divisions: Starting from one cell, after N divisions there are 2N cells. For example, after 10 divisions: 210 = 1,024 cells. If starting from M cells: total = M × 2N.
Probability of mutations: If the mutation rate per cell division is 1 in 10,000 (1 × 10⁻⁴), and 220 = 1,048,576 cells are produced, the expected number of mutations ≈ 1,048,576 × 10⁻⁴ ≈ 105. This shows why even rare mutations become significant across many cell divisions.
⚠️ Common Misconceptions
Watch Out!
Students often think mitosis produces identical cells so there is never any variation. Wrong: mitosis always produces completely identical cells with no differences whatsoever Correct: mitosis produces genetically identical cells but rare mutations during DNA replication can introduce variation
Students often think stem cells can become any type of cell forever. Wrong: stem cells remain fully flexible and can become any cell type at any time Correct: stem cells lose potency as they differentiate; once differentiated, cells cannot normally return to being stem cells
✍️ 6-Mark Question
Extended Answer Question
6 marks: Discuss the advantages and disadvantages of using embryonic stem cells compared to adult stem cells in medical treatments.
Embryonic stem cells are totipotent and can differentiate into any cell type, making them more versatile for treating a wider range of conditions. Adult stem cells are only multipotent — they can form a limited range of cell types related to their tissue of origin, restricting their medical use. However, embryonic stem cells raise significant ethical concerns because they require the destruction of an embryo, which some people believe is a potential human life. Adult stem cells avoid this ethical issue as they can be harvested from adult tissue with consent. Embryonic stem cells also have a higher risk of forming tumours (teratomas) if not properly controlled, whereas adult stem cells are more stable. Using adult stem cells from the patient's own body (autologous transplant) avoids immune rejection, whereas embryonic stem cells from a donor may be rejected by the patient's immune system unless therapeutic cloning is used.
Mark scheme: 1 mark for each advantage/disadvantage identified (max 4). 1 mark for a valid comparison between the two types. 1 mark for a balanced discussion presenting both sides.
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A study investigated stem cell therapy for spinal cord injuries. Group A received embryonic stem cell treatment, Group B received adult stem cell treatment, and Group C received no stem cell treatment (control). After 12 months, motor function improvement was: Group A — 68% showed improvement; Group B — 31% showed improvement; Group C — 8% showed improvement. Each group had 50 patients. Evaluate the evidence and explain which treatment is more effective, discussing limitations of the study.
Approach: Both stem cell treatments outperformed the control, suggesting stem cells have a real effect. Embryonic stem cells were more than twice as effective as adult stem cells (68% vs 31%), likely due to their greater potency. However, limitations include: small sample size (50 per group), short follow-up period (12 months), no information on side effects or tumour formation, and no detail on how improvement was measured. The study does not address long-term safety of embryonic stem cells, which is a key concern.
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