AQA GCSE Biology 8461: Cell division – Revision Notes

These are condensed recall notes for section 4.1.2 Cell division (4.1.2.1 to 4.1.2.3) of the AQA GCSE Biology (8461) specification, for teaching from September 2016 and exams from 2018 onwards. The content is assessed on Paper 1, set at Foundation and Higher Tier. Nothing in 4.1.2 is marked (HT only), so every line here applies to both tiers.

For full explanations and worked examples, use the Cell division study guide. Then test yourself with the Cell division practice questions. For the rest of Topic 1 (cell structure, transport), see the Cell biology revision notes. The course hub is AQA GCSE Biology, and the printable checklist lists every point.

4.1.2.1 Chromosomes – three facts

Fact	Write it like this
Where	The nucleus contains chromosomes
What they are made of	Chromosomes are made of DNA molecules
What they carry	Each chromosome carries a large number of genes
How they are arranged	In body cells, chromosomes are normally found in pairs

Size order, largest first: cell → nucleus → chromosome → gene. A gene is a section of the DNA in a chromosome.

Human body cells: 23 pairs = 46 chromosomes. Mitosis keeps the number the same in each new cell.

4.1.2.2 The cell cycle – three stages

AQA wants the three overall stages. Do not name the phases of mitosis – they are not required.

STAGE 1  Growth and DNA replication
         - cell grows
         - number of sub-cellular structures increases
           (ribosomes, mitochondria)
         - DNA replicates -> two copies of each chromosome

STAGE 2  Mitosis
         - one set of chromosomes pulled to each end of the cell
         - nucleus divides

STAGE 3  Cytoplasm and cell membrane divide
         - two genetically identical cells

Summary sentence to learn: during the cell cycle, the genetic material is doubled and then divided into two identical cells.

Method: describing the cell cycle for 4-6 marks

1. Say the cell grows and makes more ribosomes and mitochondria.
2. Say the DNA replicates so there are two copies of each chromosome.
3. Say one set of chromosomes is pulled to each end of the cell.
4. Say the nucleus divides.
5. Say the cytoplasm and cell membrane divide.
6. Finish with two genetically identical cells.

Six separate, ordered points. Each one is a likely marking point.

DNA content per cell through one cycle

Point in cycle	DNA (relative)
Start of stage 1	1
After replication	2
During mitosis	2 (in two sets)
Each new cell after stage 3	1

On a graph: flat, rise (replication), flat, sudden drop (cell splits into two).

Why mitosis matters

• Growth and development of multicellular organisms (a fertilised egg divides by mitosis to form an embryo, then cells differentiate).
• Repair and replacement of cells – in mature animals, cell division is mainly restricted to this.
• Asexual reproduction – only mitosis is involved, so offspring are clones.

Mitosis or not? Quick decision

Ask: is a eukaryotic cell making genetically identical copies of itself?

Yes – mitosis	No – not mitosis
Skin healing after a cut	Making sperm or egg cells (meiosis)
Root tip or shoot tip growing	A bacterium dividing (binary fission)
Bone marrow making blood cells
An embryo increasing its cell number
A strawberry runner forming a new plant

Data reminders

• Percentage of cells in mitosis = (cells in mitosis ÷ total cells counted) × 100.
• Time in mitosis ≈ fraction of cells in mitosis × length of the whole cycle.
• Cells after n cycles from one cell = 2ⁿ (not 2 × n).

4.1.2.3 Stem cells

Definition (AQA wording)

A stem cell is an undifferentiated cell of an organism which can give rise to many more cells of the same type, and from which certain other cells can arise by differentiation.

Three sources compared

Source	Can become	Main job
Human embryo	Most types of human cell (can be cloned and made to differentiate)	Forms the tissues of the developing body
Adult bone marrow	Many types, including blood cells	Replaces cells
Plant meristem (tips of shoots and roots)	Any type of plant cell, throughout the plant’s life	Growth

Most / many / any: learn the three words in that order.

Medical use

• Stem cell treatment may be able to help conditions such as diabetes and paralysis.
• Therapeutic cloning: an embryo is produced with the same genes as the patient → its stem cells are not rejected by the patient’s body → they may be used for medical treatment.
• Stem cell techniques are not required at GCSE.

Risks and objections

Type	Point
Practical risk	Transfer of viral infection
Ethical / religious	Some people object to producing and using human embryos

Evaluate = both sides + a conclusion linked to them.

Plant cloning from meristems

Clones are produced quickly and economically. Two uses to quote:

1. Rare species cloned to protect them from extinction.
2. Crop plants with special features (e.g. disease resistance) cloned to give large numbers of identical plants for farmers.

Small worked reminders

Mitosis in context. “A lizard regrows part of its tail. Explain the role of mitosis.”

cells near the wound divide by mitosis          -> more cells
new cells are genetically identical to parent   -> same genes
the cells then differentiate                    -> tissues of the tail

Name the process, say what it produces (identical cells), then link to the context (replacing the lost tissue).

Reading a DNA graph. If DNA per cell goes 5 → 10 → 5 units, the rise is DNA replication in stage 1 and the fall is the cell splitting in stage 3. The value never drops below the starting 5.

Stem cell source choice. “Why might a doctor prefer embryonic to bone marrow stem cells for a nerve injury?” Embryonic stem cells can differentiate into most types of human cell; bone marrow stem cells form many types, including blood cells, so their range is narrower.

Evaluate skeleton (stem cells).

benefit 1   may help conditions such as diabetes or paralysis
benefit 2   therapeutic cloning -> cells not rejected
risk 1      viral infection may be transferred
risk 2      ethical or religious objections to using embryos
conclusion  one side outweighs the other, because ... (use your points)

Plant cloning in context. “A rare orchid has only a few plants left. Suggest how meristems could help.” Take meristem tissue from a tip, grow many genetically identical clones quickly and cheaply, and so increase the number of plants and protect the species from extinction.

Must-know distinctions

Pair	Difference
Chromosome vs gene	A chromosome is a DNA molecule carrying many genes; a gene is one section of it
DNA replication vs mitosis	Replication copies the DNA (stage 1); mitosis separates the copies (stage 2)
Nucleus dividing vs cell dividing	The nucleus divides in mitosis; the cytoplasm and membrane divide afterwards
Embryonic vs adult stem cells	Most cell types vs many cell types (including blood cells)
Mitosis vs binary fission	Eukaryotic cells (nucleus) vs bacteria (no nucleus)
Therapeutic cloning vs plant cloning	Embryo with patient’s genes, for treatment vs meristem cells, to copy whole plants

Quick self-test

1. What are chromosomes made of?
2. State where chromosomes are found in a cell.
3. A body cell of one animal species has 78 chromosomes. How many pairs is that?
4. Name two sub-cellular structures whose number increases before a cell divides.
5. In which stage of the cell cycle does the DNA replicate?
6. What divides during mitosis itself?
7. One cell goes through three complete cell cycles, and every new cell also divides. How many cells result?
8. In a 24-hour cell cycle, a cell spends 3 hours in mitosis. What percentage of the cycle is this?
9. A student counts 50 cells in a root tip and 4 are in mitosis. What percentage is this?
10. Give one situation where cells divide but not by mitosis.
11. Which can form more types of cell: adult bone marrow stem cells or meristem cells?
12. Why are cells from therapeutic cloning not rejected?

Answers

1. DNA molecules.
2. In the nucleus.
3. 78 ÷ 2 = 39 pairs.
4. Ribosomes and mitochondria.
5. Stage 1, before mitosis (growth and DNA replication).
6. The nucleus (one set of chromosomes is pulled to each end of the cell).
7. 2³ = 8 cells.
8. 3 ÷ 24 × 100 = 12.5%.
9. 4 ÷ 50 × 100 = 8%.
10. Gamete (sperm or egg) production, by meiosis; or a bacterium dividing, by binary fission.
11. Meristem cells – they can differentiate into any type of plant cell.
12. The embryo they come from has the same genes as the patient, so the patient’s body does not reject them.

Where marks are usually lost

• Placing DNA replication inside mitosis instead of in the first stage.
• Leaving out the growth step: the cell grows and increases its ribosomes and mitochondria first.
• Writing “the cell divides” for stage 2 when the mark needs “the nucleus divides”.
• Naming phases of mitosis and running out of time, when three overall stages are all AQA asks for.
• Calling the two new cells “similar” – the answer needs “genetically identical”.
• Saying mitosis halves the chromosome number.
• Saying adult stem cells can become any cell, or that meristems only make root cells.
• Claiming stem cells “cure” diabetes or paralysis instead of “may be able to help”.
• Stopping a therapeutic cloning answer at “same genes” without “not rejected”.
• In an evaluate question, listing points with no conclusion.

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).