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

This guide covers 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, so everything here is for every candidate. It teaches reproduction, chromosomes, genes and alleles, mitosis, meiosis, fertilisation, differentiation, stem cells, therapeutic cloning, and tumours. Use the course hub and the printable checklist to track your progress, and take a free diagnostic to find weak spots.

The matching revision notes and practice questions go with this page. For cells, tissues and organs, see Organisation.

What this unit covers

Section	What you must be able to do
3.5.1 a	Compare sexual and asexual reproduction; define a clone
3.5.2 a-c	Link chromosomes, genes and alleles; know chromosomes occur in pairs in body cells
3.5.2 d-e	Describe mitosis and why it happens
3.5.2 f-h	Describe meiosis and the gametes it makes
3.5.2 i-j	Explain fertilisation, then mitosis, then differentiation; compare animals and plants
3.5.2 k-m	Describe stem cells, therapeutic cloning and possible treatments; judge ethical issues
3.5.2 n-o	Describe benign and malignant tumours and what can cause them

3.5.1 Reproduction: sexual and asexual

Sexual reproduction is the joining (fusion) of a male gamete and a female gamete. The offspring receive a mixture of genetic information from two parents, so they vary. Half of their genes come from each parent.

Asexual reproduction involves no fusion of gametes, and one parent is enough. There is no mixing of genetic information, so there is no genetic variation in the offspring. Genetically identical individuals are called clones.

Feature	Sexual	Asexual
Parents needed	Two	One
Gametes fuse	Yes	No
Genetic information	Mixed from both parents	All from the one parent
Offspring	Genetically different from each other	Clones of the parent

Two clones can still look different if they grow in different conditions. A plant cutting given less light than its parent may grow smaller. That difference is environmental, not genetic.

3.5.2 a-c: Chromosomes, genes and alleles

The nucleus of a cell contains chromosomes. Chromosomes carry genes, and genes control the characteristics of the body. Each chromosome carries a large number of genes, so a chromosome is not the same thing as a gene.

Many genes come in different forms called alleles, which may produce different characteristics. For example, a mouse has a gene for fur colour. One allele may give black fur and another may give brown fur. Both alleles sit at the same place on the chromosome pair.

A normal body cell holds its chromosomes as pairs. One chromosome of each pair came from each parent.

Species	Chromosomes in a body cell	Pairs	Chromosomes in a gamete
Human	46	23	23
Horse	64	32	32
Garden pea	14	7	7
Maize	20	10	10

Worked example 1. A horse has 64 chromosomes in each body cell. State the number of pairs, the number in a sperm cell, and the number in the fertilised egg.

• Pairs = 64 ÷ 2 = 32 pairs.
• A gamete has a single set, so a sperm has 64 ÷ 2 = 32 chromosomes.
• Fertilisation joins a sperm (32) to an egg (32): 32 + 32 = 64 chromosomes, the full number again.

3.5.2 d-e: Mitosis

Body cells divide by mitosis to produce additional cells during growth, or replacement cells when tissue is damaged or worn out.

When a body cell divides by mitosis:

1. Copies of the genetic material are made.
2. The cell then divides once to form two genetically identical body cells.

Each new cell ends up with the full chromosome number of the cell that divided. The specification asks for this outcome and its purpose, with no stage names.

Worked example 2. A fertilised horse egg divides by mitosis. Assume every cell divides at the same time each round.

(a) How many cells after 6 rounds of division? Each round doubles the number: 2⁶ = 64 cells.

(b) How many chromosomes are there in total across those cells? Each cell has 64, so 64 × 64 = 4096 chromosomes.

(c) How many rounds are needed for the cell number to exceed 1000? 2⁹ = 512 is too few and 2¹⁰ = 1024 is enough, so 10 rounds.

This works because every cell receives a complete, identical copy of the genetic material.

3.5.2 f-h: Meiosis and gametes

Cells in reproductive organs divide to form gametes. In animals these organs are the testes and ovaries. A cell divides to form gametes by meiosis.

When a cell divides to form gametes:

1. Copies of the genetic information are made.
2. The cell then goes through two divisions, giving four gametes. Each gamete carries a single set of chromosomes.

	Mitosis	Meiosis
Where	Body cells	Reproductive organs
Purpose	Growth, replacement	Make gametes
Divisions	One	Two
Cells made	Two	Four
Chromosomes per new cell	Full set, in pairs	Single set
Daughter cells	Genetically identical	Gametes that are not all the same

The last row goes slightly beyond the wording of the specification, but it follows from sexual reproduction giving variety. Gametes from the same parent are not copies of each other.

Worked example 3. A mouse has 40 chromosomes in a body cell. Twenty-five cells in a testis start meiosis.

• Each cell makes 4 gametes, so 25 × 4 = 100 sperm.
• Each sperm has 40 ÷ 2 = 20 chromosomes.
• Had those cells divided by mitosis, 25 cells would make 50 cells, each with 40 chromosomes.

3.5.2 i: Fertilisation and development

Gametes join at fertilisation to form a single body cell with new pairs of chromosomes. One chromosome in each pair comes from the sperm and one from the egg, which is how each parent gives half of the genes.

This cell then divides repeatedly by mitosis to form many cells. As the organism develops, these cells differentiate to form different kinds of cells. Differentiation means a cell becomes specialised for a job, for example:

• a red blood cell, which carries oxygen
• a nerve cell, which carries electrical impulses
• a root hair cell, which absorbs water and mineral ions

All these cells have the same genes. They differ because different genes are active in each type.

3.5.2 j: Animals and plants compared

In animals, most cell types become specialised early in development, and then mostly stay that way. Once an animal is mature, dividing is largely limited to repair and replacement, for example of skin or of the gut lining.

Plants differ: many of their cells can still differentiate at any age. This is why a piece of stem or shoot cut from a plant can grow new roots and become a whole new plant, and why the new plant is a clone of the parent.

3.5.2 k-m: Stem cells and therapeutic cloning

Stem cells from human embryos and from adult bone marrow can be made to differentiate into many different types of human cell. Knowledge of the techniques used is not required.

Therapeutic cloning makes an embryo that carries the patient’s own genes. The patient’s body does not reject its stem cells, because the genes match, so they can be used in treatment.

Stem cell treatment might help with paralysis, for example, where nerve cells have been damaged and the body cannot replace them.

Worked example 4. A laboratory starts a stem cell culture with 400 cells. Each cell divides once every 18 hours and none die.

(a) After 72 hours: 72 ÷ 18 = 4 divisions, so 400 × 2⁴ = 400 × 16 = 6400 cells.

(b) When does the culture first exceed 50 000 cells? We need 400 × 2ⁿ > 50 000, so 2ⁿ > 125. 2⁶ = 64 is too small and 2⁷ = 128 is enough, so n = 7. Time = 7 × 18 = 126 hours, giving 400 × 128 = 51 200 cells.

Making ethical judgements

The specification expects you to make informed judgements, when given appropriate information, about social and ethical issues in using stem cells from adult bone marrow and from embryos. Judge from the information you are given, and give at least one point on each side.

Point	Embryo stem cells	Adult bone marrow stem cells
Possible benefit	May treat conditions such as paralysis	May treat conditions such as paralysis
Concern	An embryo is used and not allowed to develop	A donor must give the cells, and this involves a procedure
Who may object	People who regard an embryo as a human life	Fewer people, but some may worry about the donor

Do not just say “it is wrong”. Say who is affected and why, then reach a conclusion that follows from your points.

3.5.2 n-o: Tumours and their causes

A tumour results from the abnormal, uncontrolled growth of cells. In other words, cells divide by mitosis without the normal control.

• Benign tumours do not invade other tissues.
• Malignant tumours have cells that invade healthy tissue. Some malignant cells may enter the bloodstream and circulate to other parts of the body, forming secondary tumours.

Tumours can be caused by:

• chemical carcinogens, for example those found in tobacco smoke and in asbestos
• ionising radiation, for example UV and X-rays

A carcinogen is something that can cause cancer. Reducing exposure, for example by not smoking and protecting skin from UV, reduces risk.

Worked example 5. A scan shows a tumour of diameter 8 mm. Nine months later it is 17 mm.

• Increase = 17 − 8 = 9 mm.
• Percentage increase = 9 ÷ 8 × 100 = 112.5%.
• Mean rate = 9 ÷ 9 = 1.0 mm per month.

Growth alone does not show a tumour is malignant. Secondary tumours in other organs would be the key evidence.

Worked example 6. Illustrative values: lung cancer cases per 100 000 people per year are 12 in non-smokers and 168 in heavy smokers. The ratio is 168 ÷ 12 = 14. Heavy smokers have 14 times the rate. This shows a strong link, and tobacco smoke contains chemical carcinogens that offer a reason why.

Common errors

• Writing that a gene and a chromosome are the same, or that a chromosome carries one gene.
• Saying meiosis makes two cells, or that mitosis makes four.
• Saying a gamete has “half a chromosome” instead of half the number of chromosomes, one from each pair.
• Describing a benign tumour as harmless in every case. The specification only says it does not invade other tissues.
• Writing “the stem cells have the patient’s DNA so they are the same as the patient’s cells” without saying why this stops rejection.
• Giving only one side of an ethical question.

Next steps

Work through the revision notes, then the practice questions. The exam preparation guide covers command words.

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.