AQA GCSE (9-1) Biology: Cell Division

Key Takeaways: Cell Division

1. Chromosomes & DNA

• Chromosomes:
  • Found in the nucleus; made of coiled DNA.
  • Humans have 46 chromosomes (23 pairs: one set from each parent).
  • Diploid cells (e.g., body cells) have 2 sets; haploid cells (e.g., gametes) have 1 set.
• Genes & Alleles:
  • A gene is a DNA section coding for a protein (e.g., eye colour).
  • Alleles are different versions of the same gene (e.g., blue vs. brown eyes).

2. Mitosis

• Purpose:
  • Growth, repair, and asexual reproduction.
  • Produces two identical diploid daughter cells.
• Stages:
  1. Interphase: Cell grows, DNA replicates.
  2. Prophase: Chromosomes condense; nuclear membrane breaks down.
  3. Metaphase: Chromosomes line up at the cell’s equator.
  4. Anaphase: Chromosomes split; chromatids pulled to opposite poles.
  5. Telophase: Nuclear membranes reform.
  6. Cytokinesis: Cytoplasm divides, forming two cells.
• Key Terms:
  • Chromatid: One half of a duplicated chromosome.
  • Cytokinesis: Division of the cytoplasm.

3. Stem Cells

• Types:
  • Embryonic stem cells: Totipotent (can differentiate into any cell type).
  • Adult stem cells: Multipotent (limited to specific cells, e.g., blood cells in bone marrow).
• Uses:
  • Medical therapies (e.g., regenerating nerve cells, treating diabetes).
  • Therapeutic cloning: Using a patient’s own stem cells to avoid rejection.
• Ethical Issues:
  • Embryonic stem cell research involves destroying embryos, which some view as unethical.

4. Plant Stem Cells & Cloning

• Meristems: Regions in plant shoots/roots where stem cells divide.
• Cuttings:
  • A stem cutting treated with rooting hormone develops into a clone (genetically identical plant).
  • No genetic variation, but environmental factors (e.g., light, soil) cause differences.

5. Key Terminology

• Gametes: Haploid sex cells (sperm, ova).
• Differentiation: Process where cells specialise for specific functions.
• Clone: Genetically identical organism/cell (e.g., plant cuttings).

6. Chromosome Numbers & Hypotheses

• Diploid Numbers Vary:
  • Humans: 46, fruit flies: 8, goldfish: 94.
  • No link between chromosome number and organism complexity.
• Evolutionary Hypothesis:
  • Chromosome fusion (e.g., human chromosome 2 formed from two ancestral chromosomes).

7. Exam Tips

• Describe Mitosis: Use stages (IPMATC) and mention identical diploid cells.
• Stem Cell Ethics: Compare embryonic vs. adult stem cells; discuss both medical benefits and ethical concerns.
• Diploid vs. Haploid: Gametes are haploid to ensure fertilisation restores diploid number (23 + 23 = 46).

Practice Questions:

• Q: Why are gametes haploid?
  A: To ensure fertilisation produces a diploid zygote (maintains chromosome number).
• Q: Name two uses of mitosis.
  A: Growth and tissue repair.

Key Diagram:

• Mitosis Stages (Prophase → Telophase) – ensure you can label chromosomes, spindle fibres, and centromeres.

50 GCSE Biology Questions on Cell Division

Answers provided at the end.

Section 1: Chromosomes & DNA

1. Define a diploid cell.
2. How many chromosomes are present in a human skin cell?
3. What is the difference between a gene and an allele?
4. Explain why human chromosomes are arranged in pairs.
5. Name the structure that holds two chromatids together.
6. What term describes a cell with half the normal chromosome number?
7. Which human cells are haploid?
8. Define allele using an example.
9. Where in a eukaryotic cell is DNA found?
10. Why do chromosomes condense during mitosis?

Section 2: Mitosis

11. List the six stages of the cell cycle in order.
12. What is the purpose of mitosis?
13. Describe what happens during anaphase.
14. How many daughter cells are produced in mitosis, and are they genetically identical?
15. Explain why mitosis is essential for growth.
16. What happens during interphase?
17. Define cytokinesis.
18. Why is it important for DNA to replicate before mitosis?
19. Name two types of cells in the human body that are replaced by mitosis.
20. Draw a simple diagram showing chromosomes aligning at the equator during mitosis.

Section 3: Stem Cells

21. What is a stem cell?
22. Distinguish between embryonic and adult stem cells.
23. Why are embryonic stem cells described as totipotent?
24. Give two medical uses of stem cells.
25. What is therapeutic cloning?
26. Why might adult stem cells be less useful than embryonic stem cells?
27. Name an animal that can regenerate lost body parts.
28. What ethical issue is associated with embryonic stem cell research?
29. How does therapeutic cloning prevent rejection of transplanted cells?
30. Where are adult stem cells found in humans?

Section 4: Plant Stem Cells & Cloning

31. Where are plant stem cells located?
32. What is a meristem?
33. Explain how plant cuttings develop into clones.
34. Why do plant clones show environmental variation?
35. What role do hormones play in plant cuttings?
36. Name a plant grown commercially using cuttings.
37. Why are plant stem cells considered more flexible than adult human stem cells?
38. Define clone in the context of plants.
39. What is rooting powder, and how does it work?
40. Why might a gardener take cuttings instead of planting seeds?

Section 5: Chromosome Numbers & Hypotheses

41. What is the diploid chromosome number of a red fox?
42. Why did scientists originally hypothesise that complex organisms have more chromosomes?
43. How does the chromosome number of humans compare to that of great apes?
44. What evidence supports the hypothesis that human chromosome 2 resulted from fusion?
45. Why was the original hypothesis linking chromosome number to complexity rejected?

Section 6: Ethics & Applications

46. Give two arguments for embryonic stem cell research.
47. Give two arguments against embryonic stem cell research.
48. What is IVF, and how is it linked to stem cell research?
49. Why might some people view embryos used in research as unethical?
50. How could stem cells potentially treat spinal cord injuries?

Detailed Answers

Section 1: Chromosomes & DNA

1. Diploid cell: A cell with two sets of chromosomes (one from each parent). Humans have 46 chromosomes (23 pairs).
2. 46 chromosomes (23 pairs).
3. A gene is a DNA segment coding for a protein. An allele is a variant of a gene (e.g., blue vs. brown eyes).
4. Chromosomes are paired because one set is inherited from each parent.
5. Centromere.
6. Haploid.
7. Gametes (sperm and ova).
8. Alleles are different versions of the same gene (e.g., gene for flower colour has alleles for purple or white).
9. In the nucleus.
10. Condensation prevents DNA tangling and ensures efficient separation during division.

Section 2: Mitosis

11. Interphase → Prophase → Metaphase → Anaphase → Telophase → Cytokinesis.
12. Growth, repair, and asexual reproduction.
13. Anaphase: Chromatids separate and move to opposite poles.
14. Two identical diploid cells.
15. Mitosis increases cell number, enabling organism growth.
16. Interphase: Cell grows, DNA replicates, organelles double.
17. Cytokinesis: Division of cytoplasm to form two cells.
18. Ensures each daughter cell has a full set of DNA.
19. Skin cells and intestinal lining cells.
20. [Diagram: Chromosomes lined up along the cell’s equator with spindle fibres attached.]

Section 3: Stem Cells

21. Stem cell: Undifferentiated cell capable of specialising into other cell types.
22. Embryonic: Totipotent (any cell type); Adult: Multipotent (limited to specific cells).
23. They can differentiate into any cell type in the body.
24. Treating paralysis or regenerating insulin-producing cells in diabetes.
25. Creating patient-matched stem cells to avoid immune rejection.
26. Adult stem cells are restricted to certain cell types.
27. Starfish or lizards.
28. Destruction of embryos is viewed as ending a potential life.
29. Patient’s own DNA is used, so immune system doesn’t attack cells.
30. Bone marrow or liver.

Section 4: Plant Stem Cells & Cloning

31. Meristems (root and shoot tips).
32. Meristem: Plant region with rapidly dividing stem cells.
33. Stem cells in cuttings differentiate into roots/shoots, creating a genetically identical plant.
34. Differences in environment (light, nutrients) affect growth.
35. Rooting hormones stimulate root development.
36. Orchids or roses.
37. Plant stem cells remain totipotent throughout life.
38. Clone: Genetically identical organism produced asexually.
39. Rooting powder contains hormones (auxins) to accelerate root growth.
40. Faster growth and identical traits to the parent plant.

Section 5: Chromosome Numbers & Hypotheses

41. 34 (diploid).
42. Assumed more complex organisms needed more genes/DNA.
43. Humans: 46; great apes: 48 (due to ancestral chromosome fusion in humans).
44. Telomere sequences and inactive centromere on human chromosome 2 match two ape chromosomes.
45. Goldfish (94) and hedgehogs (90) have more chromosomes than humans.

Section 6: Ethics & Applications

46. For: Potential cures for diseases; reduced transplant rejection.
47. Against: Embryo destruction; ethical concerns about “playing God”.
48. IVF: Fertilising eggs outside the body; unused embryos provide stem cells.
49. Belief that life begins at conception, so embryo destruction is unethical.
50. Stem cells could differentiate into nerve cells to repair damaged spinal cords.

Study Tip: Use flashcards for terms like haploid, cytokinesis, and meristem. Practice drawing mitosis stages! 🌟