AQA GCSE Biology 8461: Cell division – Study Guide

This study guide teaches section 4.1.2 Cell division (4.1.2.1 Chromosomes, 4.1.2.2 Mitosis and the cell cycle, 4.1.2.3 Stem cells) of the AQA GCSE Biology (8461) specification, for teaching from September 2016 and exams from 2018 onwards. Cell biology is assessed on Paper 1 (1 hour 45 minutes, 100 marks, 50% of the GCSE), which is set at Foundation and Higher Tier. None of the statements in 4.1.2 is marked (HT only), so everything here is needed on both tiers.

The topic-level Cell biology study guide gives the overview of the whole of Topic 1. This page goes deeper on cell division only. When you have worked through it, use the Cell division revision notes and the Cell division practice questions. The course hub is AQA GCSE Biology, and the printable checklist lists every point in the specification.

What this unit covers

Spec	What you must be able to do	Tier
4.1.2.1	State that the nucleus contains chromosomes made of DNA, that each chromosome carries many genes, and that chromosomes in body cells are normally in pairs	Both
4.1.2.2	Describe the three overall stages of the cell cycle, including mitosis; explain why mitosis matters for growth and development; recognise and describe situations where mitosis is occurring	Both
4.1.2.3	Define a stem cell; describe the function of stem cells in embryos, adult animals and plant meristems; describe medical uses, therapeutic cloning, risks and objections; describe how meristems are used to clone plants	Both

There is no required practical in 4.1.2. The specification suggests using models and analogies to explain how cells divide, and evaluating the risks, benefits, social and ethical issues of stem cell use.

4.1.2.1 Chromosomes

Three ideas, and you need all three:

1. The nucleus of a cell contains chromosomes made of DNA molecules.
2. Each chromosome carries a large number of genes.
3. In body cells the chromosomes are normally found in pairs.

Think of the levels from big to small: nucleus → chromosome → gene → DNA. A chromosome is one very long DNA molecule. A gene is a section of that DNA. So a single chromosome carries many genes, one after another along its length.

“In pairs” means a body cell has two copies of each kind of chromosome, one inherited from each parent. Human body cells contain 23 pairs, which is 46 chromosomes in total. You meet that number again in Topic 6 (inheritance), but it is useful here for counting questions.

Worked example 1. A body cell from a species has 18 chromosomes. How many pairs does it have, and how many chromosomes will each cell made from it by mitosis contain?

pairs = 18 / 2 = 9 pairs
mitosis gives cells genetically identical to the parent
chromosomes in each new cell = 18

The second answer does not halve. Halving happens in meiosis, which makes gametes and belongs to Topic 6.

4.1.2.2 Mitosis and the cell cycle

The three stages

Cells divide in a series of stages called the cell cycle. During the cycle the genetic material is doubled and then divided into two identical cells. AQA asks you to know the three overall stages. You do not need the names of the separate phases inside mitosis.

Stage	What happens
1. Growth and DNA replication	The cell grows. It increases the number of sub-cellular structures such as ribosomes and mitochondria. The DNA replicates to form two copies of each chromosome.
2. Mitosis	One set of chromosomes is pulled to each end of the cell and the nucleus divides.
3. Division of cytoplasm and membrane	The cytoplasm and cell membranes divide to form two identical cells.

Why the first stage matters: each new cell needs a full set of genetic material and enough organelles to survive. If DNA did not replicate first, each new cell would get only half the chromosomes. If ribosomes and mitochondria did not increase, the new cells would be short of protein synthesis and respiration.

A useful model: photocopy every book in a library (DNA replication), buy more shelves (more organelles), move one complete set to each end of the building (mitosis), then build a wall down the middle (stage 3).

Tracking the DNA through one cycle

Worked example 2. A cell has 8 units of DNA at the start of the cycle. Describe how the DNA content of the cell changes through one complete cycle.

start of stage 1:      8 units
after DNA replication: 16 units (two copies of each chromosome)
during mitosis:        16 units, now in two separate sets
after stage 3:         two cells, each with 8 units

The DNA doubles and then halves back to the starting value in each new cell. On a graph of DNA content per cell against time, this shows as a rise during stage 1, a flat section, then a sudden drop to the starting value when the cell splits.

Why mitosis is important

Cell division by mitosis is important in the growth and development of multicellular organisms. A multicellular organism starts as one cell. After fertilisation, the new cell divides by mitosis, the number of cells increases, and as the embryo develops the cells differentiate. Mitosis also replaces cells, for example when a wound heals.

Recognising where mitosis is happening

The specification says you must recognise and describe situations in given contexts where mitosis is occurring. The test is: are new cells being made that are genetically identical to the parent cell, in a eukaryotic organism?

Context	Mitosis?	Why
A cut on the skin healing	Yes	New skin cells replace damaged ones
An embryo growing from a fertilised egg	Yes	Cell number increases for growth
Cells dividing just behind a root tip	Yes	The root grows longer
Bone marrow making new blood cells	Yes	Stem cells divide to replace blood cells
A plant cutting growing into a new plant	Yes	Asexual reproduction uses only mitosis
Testes making sperm	No	Gametes are made by meiosis
A bacterium splitting into two	No	Bacteria have no nucleus and divide by binary fission

Worked example 3 (data skill). A student counts the cells in one area of a stained onion root tip. She counts 400 cells. In 36 of them, chromosomes are visibly being pulled apart, so the cell is in mitosis. The cell cycle in this tissue takes 20 hours. Estimate how long a cell spends in mitosis.

fraction in mitosis = 36 / 400 = 0.09   (9%)
time in mitosis     = 0.09 × 20 h = 1.8 h
convert             = 1.8 × 60 = 108 minutes

This works because, if cells are dividing at random times, the fraction of cells you see in a stage matches the fraction of the cycle spent in that stage. Always show the fraction or percentage before you multiply.

Worked example 4 (data skill). A single cell divides by mitosis every 12 hours. Each new cell also divides every 12 hours. How many cells are there after 72 hours?

number of cycles = 72 / 12 = 6
cells = 2⁶ = 64

The number doubles each cycle. A common slip is to calculate 2 × 6 = 12.

4.1.2.3 Stem cells

Definition

Learn the AQA wording: a stem cell is an undifferentiated cell of an organism which is capable of giving rise to many more cells of the same type, and from which certain other cells can arise from differentiation.

Two features are tested: it is not yet specialised, and it can both make more of itself and produce specialised cells.

Function in embryos, adult animals and plants

Where	Function	What they can become
Human embryos	Divide and differentiate to form all the tissues as the embryo develops	Can be cloned and made to differentiate into most different types of human cells
Adult animals (e.g. bone marrow)	Replace cells that are lost or damaged	Many types of cells including blood cells, but a narrower range than embryonic stem cells
Plant meristems	Let the plant grow at the tips of shoots and roots for its whole life	Any type of plant cell, throughout the life of the plant

Link this to differentiation in 4.1.1.4: most animal cells differentiate at an early stage, so in a mature animal, cell division is mainly for repair and replacement. Many plant cells keep the ability to differentiate throughout life.

You do not need to know the laboratory techniques for growing or treating stem cells. The specification says so directly.

Medical uses

Treatment with stem cells may be able to help conditions such as diabetes and paralysis. Note the wording “may be able to”: write about these as possible treatments, not established cures. For example, stem cells might be made to differentiate into insulin-producing cells for a person with diabetes, or into nerve cells for a person with paralysis.

Therapeutic cloning

In therapeutic cloning an embryo is produced with the same genes as the patient. Stem cells from that embryo are not rejected by the patient’s body, so they may be used for medical treatment.

Worked example 5. Explain why stem cells from therapeutic cloning are less likely to be rejected than stem cells from an unrelated donor embryo.

A full answer has a chain of three links:

1. The embryo is produced with the same genes as the patient.
2. So the stem cells, and any cells made from them, are genetically identical to the patient’s own cells.
3. So the patient’s body does not recognise them as foreign, and does not reject them.

Stopping after link 1 is the usual way to drop marks. You must get to “not rejected”.

Risks and objections

• Practical risk: transfer of viral infection from the stem cells or the culture to the patient.
• Ethical or religious objections: some people object to making and destroying human embryos for research or treatment.

When asked to evaluate stem cell use, give benefits and risks, then a justified conclusion. Benefits include possible treatment of conditions with few other options, and (with therapeutic cloning) no rejection. Risks include viral infection and the ethical and religious objections above. A conclusion such as “the benefits may outweigh the risks if cells are screened for viruses, but people’s objections mean it should be regulated” earns credit because it is linked to the points you made.

Cloning plants from meristems

Stem cells from meristems can be used to produce clones of plants quickly and economically. The specification gives two uses:

• Rare species can be cloned to protect them from extinction.
• Crop plants with special features, such as disease resistance, can be cloned to produce large numbers of identical plants for farmers.

The clones are genetically identical because they are produced by mitosis. That is the advantage for a crop (every plant has the useful feature) and a weakness too (they are all equally exposed to any new disease that the feature does not protect against).

Common errors

• Writing “the chromosomes replicate in mitosis”. DNA replication happens in the first stage, before mitosis.
• Describing four or five named phases of mitosis instead of the three overall stages AQA asks for.
• Forgetting that the cell grows and makes more ribosomes and mitochondria before dividing.
• Saying mitosis makes cells with half the chromosomes. That is meiosis.
• Saying a bacterium divides by mitosis. Bacteria use binary fission.
• Saying adult stem cells can form any cell type. Bone marrow stem cells form many types, including blood cells. Embryonic stem cells form most types. Only meristems form any type (of plant cell).
• Writing “stem cells cure diabetes”. The specification says they may be able to help.
• In a therapeutic cloning answer, not stating that the cells will not be rejected.

Next steps

• Condensed recall: Cell division revision notes
• Exam-style questions: Cell division practice questions
• Whole-topic questions: Cell biology practice questions
• Whole-topic notes: Cell biology revision notes
• Check what you know with a free 10-minute diagnostic

Official syllabus

AQA GCSE Biology (8461) specification, Version 1.0, for teaching from September 2016 and exams from 2018 onwards (AQA), section 4.1.2 Cell division (4.1.2.1 to 4.1.2.3).