4.1.2 Cell division

Why this matters

You started life as a single fertilised egg. Every one of your trillions of cells today is descended from that one cell by mitosis — a kind of cell division that produces two genetically identical 'daughter' cells from one 'parent'. Your body uses mitosis constantly: to grow from a baby into an adult, to repair cuts and broken bones, to replace cells that wear out (you replace your skin every few weeks and your gut lining every few days). When the controls on mitosis break, cells divide uncontrollably and form a tumour — that's cancer. Plants use mitosis too, but in special regions called meristems found at the tips of roots and shoots. Stem cells are unspecialised cells that can divide by mitosis and then DIFFERENTIATE into specific cell types — and they're the basis of some of the most promising (and controversial) treatments in modern medicine.

How to learn this topic

Build on what you already know

• KS3: cells divide to make new cells; growth requires more cells.
• GCSE 4.1.1: nucleus contains the DNA / chromosomes.
• GCSE 4.1.1: stem cells are unspecialised cells that can become any type of cell.

1. Chromosomes first — what they are, where they sit, and why they have to be copied before division.
2. The cell cycle: a slow growth phase (most of the cell's life) followed by a quick mitosis phase.
3. Mitosis stage by stage — interphase, prophase, metaphase, anaphase, telophase, cytokinesis. Don't memorise the names without understanding what happens.
4. Result: two genetically identical daughter cells.
5. Stem cells: embryonic (any cell type), adult (a few cell types), plant meristem (any plant cell). Therapeutic uses + ethical issues.
6. Why mitosis matters in real life: growth, repair, replacement, and — when it goes wrong — cancer.

Key terms

chromosome

A long, coiled strand of DNA found in the nucleus. Carries the cell's genetic information in the form of genes. (Examiners want 'chromosomes contain DNA / genes / genetic material'. Saying 'chromosomes ARE genes' is the most common misuse.)

mitosis

A type of cell division that produces two daughter cells, each genetically identical to the parent. Used for growth, repair, and asexual reproduction. (Specify 'genetically identical' AND 'daughter cells' — both phrases are usually required for the full mark.)

cell cycle

The complete sequence of stages a cell goes through between one division and the next: interphase (growth + DNA replication) followed by mitosis (nucleus division) and cytokinesis (cytoplasm division). (AQA's three-stage version: cell grows + sub-cellular structures increase → DNA copies → mitosis produces two identical cells.)

interphase

The longest stage of the cell cycle. The cell grows, makes more organelles, and copies its DNA. (Don't say mitosis happens during interphase — interphase is the PREPARATION phase before mitosis.)

cytokinesis

The final stage of cell division: the cytoplasm (and in plants the cell wall) splits, producing two separate daughter cells.

daughter cell

A new cell produced by cell division. After mitosis, both daughter cells are genetically identical to the parent. (If a question asks about meiosis, daughter cells are NOT identical — but AQA 4.1.2 is mitosis only.)

stem cell

An unspecialised cell that can divide by mitosis to produce more cells and that can differentiate into different specialised cell types. (Always state BOTH features — 'unspecialised' AND 'can differentiate'.)

embryonic stem cell

A stem cell from an early embryo. Can differentiate into any cell type in the body — sometimes called 'pluripotent'. ('Any cell type' (or 'pluripotent') is the marking phrase that distinguishes embryonic from adult stem cells.)

adult stem cell

A stem cell found in some adult tissues (e.g. bone marrow). Can only differentiate into a limited range of cell types. (Examiners want both 'found in adult tissue' AND 'limited range of cell types'. Bone marrow is the canonical example.)

meristem

A region of unspecialised, actively dividing cells in plants — found at the tips of roots and shoots. Plant equivalent of a stem-cell pool. (AQA explicitly distinguishes meristematic cells (plant) from animal stem cells. Plant meristem cells can become ANY plant cell type, throughout the plant's life.)

differentiation

The process by which an unspecialised cell becomes specialised for a particular function. Once a cell has differentiated, it usually cannot revert. (Mark schemes accept both 'becomes specialised' and 'develops into a specific cell type'.)

therapeutic cloning

Producing an embryo with the same genes as a patient, then extracting stem cells from that embryo to grow new cells/tissue for treatment — without rejection. (The 'no rejection' point is the marking phrase, because the cells share the patient's own genes.)

Notes

Chromosomes — the unit of inheritance

The nucleus contains your chromosomes — long, tightly-coiled strands of DNA. The DNA is divided into sections called genes that each code for a specific protein. Humans have 46 chromosomes (23 pairs) in every body cell. Sperm and egg cells have only 23 — so when they fuse at fertilisation, the new cell has the normal 46.

Before a cell can divide, every chromosome must be copied — otherwise the two daughter cells would end up with only half the genetic information each.

The cell cycle

Most of a cell's life is spent in interphase — a long growth phase. During interphase:

• The cell grows in size.
• The cell makes more sub-cellular structures: ribosomes, mitochondria, etc.
• The DNA is copied so each chromosome now exists as two identical sister copies (joined at a centromere).

Only after all that does the cell enter mitosis — the actual division phase.

Mitosis — the stages

Mitosis is one continuous process, but it's helpful to break it into stages:

1. Prophase — chromosomes condense (coil up) until they're visible as X-shaped pairs of sister chromatids. The nuclear envelope breaks down.
2. Metaphase — chromosomes line up across the equator (middle) of the cell.
3. Anaphase — the two sister chromatids of each chromosome are pulled apart by spindle fibres, one to each end of the cell.
4. Telophase — a new nuclear envelope forms around each set of chromosomes. The cell now has two complete nuclei.
5. Cytokinesis — the cell membrane (and in plants, the cell wall) pinches in, splitting the cytoplasm and producing two separate daughter cells.

The result: two daughter cells, each genetically identical to the parent cell.

AQA only requires you to know the cell cycle in three broad stages (cell grows, DNA replicates, mitosis), but knowing the named stages helps with 4-mark 'describe what happens' questions.

Why mitosis matters

Mitosis is used wherever the body needs more of the same kind of cell:

• Growth — turning a fertilised egg into a baby, then a baby into an adult.
• Repair — fixing a cut, knitting a broken bone, replacing damaged tissue.
• Replacement — your skin, gut lining, and blood cells are constantly being replaced.
• Asexual reproduction — some organisms (e.g. bacteria, hydra, strawberry plants via runners) reproduce without a partner, using mitosis to produce genetically identical offspring (clones).

When the controls on mitosis break, cells divide uncontrollably. The growing mass of cells is called a tumour, and the disease is cancer.

Stem cells

Most cells in your body are specialised (or differentiated) — a nerve cell is a nerve cell, and that's all it can ever be. Stem cells are unspecialised. They can:

• Divide by mitosis to make more cells.
• Differentiate into specific cell types.

There are three sources you need to know:

• Embryonic stem cells — found in early embryos (a few days old). Can differentiate into any cell type in the body — these are 'pluripotent'.
• Adult stem cells — found in some adult tissues, such as bone marrow. Can only differentiate into a limited range of cell types. Bone marrow stem cells, for example, produce all the different blood cells.
• Plant meristematic cells — found in the meristems at the tips of roots and shoots. These are the plant equivalent of stem cells: they can differentiate into any plant cell type, and plants can do this throughout their lives, which is why a cutting can grow into a whole new plant.

Therapeutic uses of stem cells

• Treating diabetes — making new insulin-producing cells from stem cells to replace damaged ones.
• Treating paralysis — regenerating nerve tissue.
• Bone marrow transplants — adult stem cells from a donor's bone marrow can produce new blood cells in a leukaemia patient.
• Plant cuttings + cloning — meristem cells let growers produce many genetically identical plants (e.g. for rare or endangered varieties, or for farming).

Ethical + practical issues

Embryonic stem cells are powerful (any cell type) but getting them destroys the embryo, which some people object to on religious or ethical grounds. There's also a risk of infection or rejection when stem cells are transplanted. Therapeutic cloning (making stem cells from a patient's own cells) is being researched as a way to avoid rejection — the cells share the patient's own genes.

Adult stem cells are less powerful (only some cell types) and are harder to grow in the lab, but they don't involve embryos so they raise fewer ethical concerns.

Exam tips

• Always specify that daughter cells are 'genetically identical' to the parent — saying just 'identical' or 'the same' often loses the mark.
• Chromosomes MUST be copied BEFORE mitosis, not during. The copying happens in interphase.
• On stem cell questions, name BOTH embryonic and adult sources and contrast them: 'embryonic = any cell type; adult = limited range'.
• For plant questions about growth, the marking phrase is 'meristem' or 'meristematic cells', not 'stem cells in plants'.
• Ethical-issue questions usually want one ethical concern (embryo destroyed) PLUS one practical concern (risk of rejection, viral transmission, or unwanted growth).
• Therapeutic cloning's main advantage is 'no rejection' because the cells share the patient's own genes — name this explicitly.
• Don't write 'cancer is caused by mitosis' — write 'cancer is caused by uncontrolled mitosis / when controls on the cell cycle break down'.

Mark-scheme phrasing

Common misconceptions

• —
• —
• —
• —

Worked example

Question:

Answer:

Frequently asked questions

Related topics in AQA GCSE Biology (8461)

• 4.1.1 Cell structure
• 4.1.3 Transport in cells