B2: Cell Division
Mitosis, the cell cycle and stem cells
B2 Cell Division
📋 Key Concepts
Cell division is how organisms grow and repair damaged tissues. New cells are produced when existing cells divide. In humans, this happens constantly - for example, your skin cells divide every few hours to replace worn-out cells.
Chromosomes are structures in the nucleus that contain DNA (genetic material). Human body cells have 46 chromosomes (23 pairs). Sex cells (gametes) have 23 chromosomes (one of each pair).
📝 The Cell Cycle
The cell cycle is the series of stages a cell goes through to grow and divide. It has three main stages: growth, DNA replication, and mitosis.
Stage 1: Growth and DNA Replication
- The cell grows and increases in size
- The number of organelles increases (e.g. more mitochondria and ribosomes)
- DNA is replicated (copied) so there are two copies of each chromosome
- The DNA is copied so each new cell will get a full set of genetic information
Stage 2: Mitosis
- The chromosomes line up at the centre of the cell
- The chromosome copies are pulled apart to opposite ends of the cell
- The nucleus divides, creating two identical nuclei
- The cell membrane and cytoplasm divide (cytokinesis)
- Result: two genetically identical daughter cells
Mitosis produces two identical daughter cells.
Each daughter cell has the same chromosomes as the parent cell - they are genetically identical clones.
Each daughter cell has the same chromosomes as the parent cell - they are genetically identical clones.
Example 1
Question: A cell with 46 chromosomes undergoes mitosis. How many chromosomes does each daughter cell have?
Answer: Each daughter cell has 46 chromosomes. Mitosis produces identical copies, so the chromosome number stays the same.
📝 Why Is Mitosis Important?
Mitosis is essential for three main purposes:
- Growth - multicellular organisms grow by producing more cells through mitosis
- Repair - damaged or worn-out cells are replaced by mitosis (e.g. skin healing after a cut)
- Asexual reproduction - some organisms reproduce by mitosis alone (e.g. bacteria, some plants)
Example 2
Question: Explain why mitosis is important for repairing a cut on your skin.
Answer: When skin is cut, cells are damaged or destroyed. Mitosis produces new, genetically identical cells that replace the damaged ones. This allows the skin to heal and return to normal function.
📝 Stem Cells
Stem cells are undifferentiated cells - they have not yet specialised for a particular function. They can divide to produce more stem cells, or differentiate into specialised cells (like nerve cells, muscle cells, blood cells).
Types of Stem Cells
| Type | Source | What They Can Become |
|---|---|---|
| Embryonic stem cells | Early-stage embryo (4-5 days old) | Any type of cell in the body (totipotent/pluripotent) |
| Adult stem cells | Body tissues (e.g. bone marrow) | Limited range of cell types related to their tissue |
Embryonic stem cells can differentiate into ANY type of cell. This makes them very useful in medicine but also raises ethical concerns because the embryo is destroyed when stem cells are harvested.
Adult stem cells (e.g. from bone marrow) can only differentiate into a limited range of cells. They are less useful but have fewer ethical issues since the donor is not harmed.
Example 3
Question: Explain the difference between embryonic stem cells and adult stem cells.
Answer: Embryonic stem cells can differentiate into any type of cell in the body, whereas adult stem cells can only differentiate into a limited range of cell types related to the tissue they come from. Embryonic stem cells come from early embryos; adult stem cells come from body tissues like bone marrow.
📝 Uses of Stem Cells
Medical Uses
- Leukaemia treatment - bone marrow transplants replace diseased blood cells with healthy ones from a donor
- Spinal cord repair - stem cells could help repair nerve damage
- Diabetes - stem cells could produce insulin-producing cells for Type 1 diabetes
- Blindness - stem cells could replace damaged retinal cells
- Drug testing - stem cells can be grown into tissues for testing new medicines
Therapeutic Cloning
Therapeutic cloning involves creating an embryo with the same DNA as the patient. Stem cells from this embryo would be a perfect genetic match, so the body would not reject them.
Example 4
Question: A patient with leukaemia receives a bone marrow transplant. Explain how this treatment works.
Answer: The patient's diseased bone marrow is destroyed using chemotherapy or radiotherapy. Healthy stem cells from a donor's bone marrow are then transplanted into the patient. These stem cells divide by mitosis and differentiate into healthy blood cells, replacing the diseased ones.
📝 Ethical Issues with Stem Cells
The main ethical issue is about embryonic stem cells. To harvest them, the embryo is destroyed. Some people believe this is wrong because the embryo could have developed into a human being.
| Arguments FOR embryonic stem cell research | Arguments AGAINST embryonic stem cell research |
|---|---|
| Could cure many serious diseases | Embryo is destroyed - some see this as taking a potential life |
| Improves quality of life for many people | Each embryo has the potential to become a human being |
| Embryos used are often from IVF and would be destroyed anyway | Adult stem cells could be used instead (fewer ethical issues) |
| Research is heavily regulated | Could lead to slippery slope of embryo exploitation |
Example 5
Question: Discuss the ethical issues relating to the use of embryonic stem cells. (4 marks)
Answer: Some people argue that using embryonic stem cells is unethical because the embryo is destroyed, and it has the potential to become a human life. However, others argue that the embryos used would often be destroyed anyway (from IVF clinics) and that the potential to cure serious diseases justifies the research. Some people believe adult stem cells should be used instead as they do not require the destruction of an embryo, but adult stem cells cannot differentiate into as many cell types.
❓ Practice Questions
Q1: Describe the stages of the cell cycle. (3 marks)
Q2: How many chromosomes are in a human body cell?
Q3: Explain why mitosis is important for growth and repair. (3 marks)
Q4: What is a stem cell?
Q5: Explain the difference between embryonic stem cells and adult stem cells. (2 marks)
Q6: Discuss the ethical issues relating to the use of embryonic stem cells in medical research. (4 marks)
✅ Answers
- The cell grows and increases the number of organelles; the DNA is replicated (copied); mitosis occurs where the nucleus divides to produce two identical nuclei, then the cytoplasm and cell membrane divide.
- 46 chromosomes (23 pairs)
- Mitosis produces new cells that are genetically identical to the parent cell. For growth, this means organisms can produce more cells to increase in size. For repair, mitosis replaces damaged or worn-out cells with identical copies that function the same way.
- A stem cell is an undifferentiated cell that has not yet specialised. It can divide to produce more stem cells, or differentiate into a specialised cell type.
- Embryonic stem cells can differentiate into any type of cell in the body, whereas adult stem cells can only differentiate into a limited range of cell types related to their tissue of origin.
- See Example 5 above - include arguments both for and against, showing both sides of the debate.
🎯 Exam Tips
- When describing the cell cycle, mention growth, DNA replication AND mitosis - all three stages
- "Two genetically identical daughter cells" - always say "identical" when describing mitosis
- Don't confuse mitosis with meiosis - mitosis produces identical cells; meiosis produces gametes
- For stem cell ethics questions, always give both sides of the argument
- Remember: bone marrow transplant = adult stem cells (not embryonic)
- Exam questions often ask you to "discuss" ethical issues - this means give arguments for AND against
🔢 Maths Skills
Mathematical Skills
Cell number after N divisions by mitosis = 2N
Starting from 1 cell: after 1 division → 2 cells; after 2 → 4; after 3 → 8; after 10 → 1024
Starting from 1 cell: after 1 division → 2 cells; after 2 → 4; after 3 → 8; after 10 → 1024
Maths Example
A single bacterial cell divides every 20 minutes. How many cells will there be after 2 hours?
2 hours = 120 minutes ÷ 20 = 6 divisions. Number = 26 = 64 cells.
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Mitosis produces identical cells so there is no variation Correct: Mitosis produces genetically identical cells, but mutations can occur during DNA replication, introducing variation
2. Wrong: Stem cells can become any cell type forever Correct: Stem cells differentiate and lose potency — adult stem cells are already partially specialised and can only form limited cell types
✍️ 6-Mark Question
Extended Answer
6 marks: Discuss the ethical issues of using embryonic stem cells compared with adult stem cells in medical research.
Embryonic stem cells can differentiate into any cell type, making them potentially more useful for treating a wide range of diseases such as paralysis and diabetes. However, to harvest embryonic stem cells the embryo is destroyed, which some people believe is unethical as the embryo has the potential to become a human life. Adult stem cells, such as those from bone marrow, avoid this ethical issue because the donor is not harmed. However, adult stem cells can only differentiate into a limited range of cell types, reducing their medical usefulness. Some people argue that embryos from IVF would be destroyed anyway, so using them for research is justified. Others worry about a slippery slope towards embryo exploitation. Therapeutic cloning could produce genetically matched embryonic stem cells but still requires embryo destruction.
Mark scheme: 1 mark for embryonic advantage; 1 mark for ethical objection to embryo use; 1 mark for adult stem cell advantage; 1 mark for adult stem cell limitation; 1 mark for IVF counter-argument; 1 mark for balanced discussion
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A study tested stem cell therapy on 50 patients with spinal cord injuries. 30 patients received stem cell treatment and 20 received a placebo. After 6 months, 18 of the treated group and 4 of the placebo group showed improvement.
(a) Calculate the percentage improvement in each group.
(b) Evaluate whether the stem cell therapy was effective.
Answers: (a) Treated: (18/30) × 100 = 60%. Placebo: (4/20) × 100 = 20%. (b) The therapy appears effective — 60% improved vs 20% in placebo. However, the groups are unequal in size (30 vs 20), and the study is small. A larger trial with equal group sizes and longer follow-up would strengthen the conclusion.
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