OxfordAQA International GCSE Biology: Reproduction, cell division, stem cells and tumours (9201) – Revision Notes

These notes condense sections 3.5.1 and 3.5.2 of the OxfordAQA International GCSE Biology (9201) specification, Version 5.2, for exams May/June 2018 onwards. The qualification is untiered. For full explanations and worked examples, read the study guide first. Then test yourself with the practice questions, tick off topics on the checklist, and see the course hub. A free diagnostic shows what to revise first.

Key definitions

Term	Meaning
Sexual reproduction	Joining (fusion) of male and female gametes; mixed genetic information from two parents leads to variety
Asexual reproduction	No fusion of gametes; one parent; no mixing, so no genetic variation
Clone	Genetically identical individual made by asexual reproduction
Chromosome	Found in the nucleus; carries many genes
Gene	Controls a characteristic of the body; many genes per chromosome
Allele	A different form of a gene; may produce a different characteristic
Mitosis	Division of a body cell into two genetically identical body cells
Meiosis	Division of a cell in a reproductive organ into four gametes
Gamete	Sex cell with a single set of chromosomes
Fertilisation	Gametes join to form a single body cell with new pairs of chromosomes
Differentiation	Cells become specialised for different jobs
Stem cell	Cell (from embryos or adult bone marrow) that can be made to differentiate into many cell types
Therapeutic cloning	Embryo carrying the patient’s genes; its stem cells are not rejected
Tumour	Abnormal, uncontrolled growth of cells
Carcinogen	Agent that can cause tumours, such as chemicals in tobacco smoke or asbestos, or ionising radiation

Sexual versus asexual (3.5.1 a)

	Sexual	Asexual
Parents	Two	One
Gametes fuse	Yes	No
Variation in offspring	Yes	None genetically
Offspring called	Offspring	Clones
Share of genes from each parent	Half	All from one

Chromosomes, genes and alleles (3.5.2 a-c)

• Chromosomes are in the nucleus and carry genes.
• A chromosome carries a large number of genes.
• Alleles are different forms of the same gene.
• Body cells have chromosomes in pairs. Gametes have one from each pair.
• Number in a body cell, then pairs, then gamete: 46, 23, 23 for a human.

Rule: pairs = body number ÷ 2, gamete number = body number ÷ 2, fertilised egg = body number.

Mitosis and meiosis side by side (3.5.2 d-h)

	Mitosis	Meiosis
Cell type	Body cell	Cell in reproductive organ
Genetic material copied first	Yes	Yes
Divisions	1	2
Cells made	2	4
Chromosomes in each new cell	Full set, in pairs	Single set
Purpose	Growth, replacement	Gametes

Method: counting cells and chromosomes

1. Decide whether the question is about mitosis or meiosis.
2. Mitosis: cells after n rounds = 2ⁿ, each with the full number.
3. Meiosis: one cell gives 4 gametes, each with half the body number.
4. Multiply by the number of starting cells if there are several.

Worked reminder: a pea body cell has 14 chromosomes. A cell undergoing meiosis gives 4 gametes with 14 ÷ 2 = 7 chromosomes each. After 5 rounds of mitosis one cell becomes 2⁵ = 32 cells.

Fertilisation, differentiation and plants (3.5.2 i-j)

Order of events:

1. Gametes fuse at fertilisation, forming a body cell with new pairs.
2. That cell divides by mitosis many times.
3. Cells differentiate into different kinds of cell as the organism develops.

	Animals	Plants
Differentiation	Most cell types specialise early	Many cells can still specialise at any age
Division in adults	Largely repair and replacement	Cells can still differentiate, so new tissues can form

Stem cells and therapeutic cloning (3.5.2 k-m)

• Sources: human embryos and adult bone marrow.
• They can be made to differentiate into many types of human cell.
• Possible use: conditions such as paralysis.
• Therapeutic cloning: the embryo carries the patient’s genes, so the cells are not rejected.
• Knowledge of stem cell techniques is not required.

Ethical judgements

Use the information given. Cover:

1. Benefit to patients (treating illness).
2. Concern about using embryos, which are not allowed to develop.
3. Concern about the donor for bone marrow cells.
4. Different views among people, including religious views.
5. A reasoned conclusion that follows from your points.

Tumours and carcinogens (3.5.2 n-o)

	Benign	Malignant
Cause	Abnormal, uncontrolled cell growth	Abnormal, uncontrolled cell growth
Invades healthy tissue	No	Yes
Cells enter bloodstream	No	Some may
Secondary tumours	No	Can form

Causes: chemical carcinogens (tobacco smoke, asbestos); ionising radiation (UV, X-rays).

Must-know distinctions

• Gene versus allele: a gene is a section of instructions for one characteristic; alleles are its forms.
• Chromosome versus gene: a chromosome carries many genes.
• Mitosis gives identical body cells; meiosis gives gametes.
• Benign tumours do not invade; malignant ones do.
• Gametes have one set; body cells have pairs.

Worked reminders

Percentage change and rate

1. Find the change: final value minus starting value.
2. Divide by the starting value and multiply by 100.
3. For a rate, divide the change by the time and give the unit.

Example: a tumour grows from 6 mm to 15 mm in 9 months. Change = 9 mm. Percentage increase = 9 ÷ 6 × 100 = 150%. Mean rate = 9 ÷ 9 = 1 mm per month.

Doubling cultures

A stem cell culture of 300 cells doubles every 12 hours. After 48 hours there are 48 ÷ 12 = 4 doublings, so 300 × 2⁴ = 4800 cells. If you are given a table, check that each row is double the one before before you extend the pattern.

Counting gametes and chromosomes

A maize body cell has 20 chromosomes. One cell undergoing meiosis makes 4 gametes with 10 chromosomes each. Two gametes fusing give 20 again. A zygote that undergoes 3 rounds of mitosis becomes 2³ = 8 cells, each with 20 chromosomes.

Facts to link together

• Asexual reproduction needs one parent, so the offspring are clones, and any differences between them come from the environment.
• Sexual reproduction mixes genetic information, because each parent supplies one set of chromosomes through a gamete.
• Fertilisation restores the pairs, mitosis builds the organism, and differentiation makes it specialised.
• Stem cells are valuable because they can be made to differentiate into many types of human cell.
• Therapeutic cloning matches the genes of the stem cells to the patient.
• A tumour is uncontrolled cell division. Whether it is benign or malignant depends on whether its cells invade healthy tissue.
• Carcinogens and ionising radiation raise the chance that cells divide without control.

Exam technique for this unit

• Explain questions need a cause and a result, joined by “because” or “so”.
• Calculate questions need working. Write the sum before the answer.
• Evaluate or make a judgement questions need points on both sides and a conclusion.
• State questions need a short answer without explanation.
• When asked about chromosomes, give numbers whenever the question supplies a number.

Quick self-test

1. A chimpanzee has 48 chromosomes in a body cell. How many pairs?
2. How many chromosomes are in a chimpanzee sperm?
3. How many cells result from one round of mitosis in one cell?
4. How many gametes result from one cell dividing by meiosis?
5. A zygote divides by mitosis 5 times, with all cells dividing together. How many cells?
6. Name two sources of stem cells that the specification mentions.
7. Why are stem cells from therapeutic cloning not rejected?
8. State one difference between benign and malignant tumours.
9. A tumour has a mass of 2.5 g and grows to 4.0 g. Calculate the percentage increase.
10. Name two chemical carcinogens or sources of carcinogens.
11. Why can a plant cutting grow roots, when a mature animal muscle cell does not form a new animal?
12. Give the term for genetically identical offspring.

Answers

1. 48 ÷ 2 = 24 pairs.
2. 24.
3. Two.
4. Four.
5. 2⁵ = 32 cells.
6. Human embryos and adult bone marrow.
7. The embryo carries the patient’s genes, so the immune system does not see the cells as foreign.
8. Malignant tumour cells invade healthy tissue; benign tumours do not.
9. (4.0 − 2.5) ÷ 2.5 × 100 = 60%.
10. Any two from tobacco smoke and asbestos.
11. Plant cells can still differentiate at any age; most animal cells specialise early and later divide mainly for repair and replacement.
12. Clones.

Where marks are usually lost

• Writing “half the chromosomes” for a body cell instead of working out the number from the data.
• Mixing up the number of cells: mitosis makes two, meiosis makes four.
• Saying gametes are “identical” to each other when comparing with mitosis.
• Calling a benign tumour “not cancerous” without saying it does not invade other tissues.
• Giving an ethical answer that lists only advantages, or only objections.
• Forgetting the percentage change formula: change ÷ original × 100.
• Writing “the embryo is the patient” instead of saying it carries the patient’s genes.
• Forgetting units in rate answers, such as mm per month.

Official syllabus

OxfordAQA International GCSE Biology (9201), specification Version 5.2, for exams May/June 2018 onwards, published by OxfordAQA. Sections 3.5.1 a and 3.5.2 a to o.