The big idea: Mitosis is the division of one nucleus into two new nuclei that are genetically identical to each other and to the original cell.
It happens in one division and always produces two daughter nuclei with the same number of chromosomes as the parent (they stay diploid).
Mitosis is used for growth, repair of damaged tissue, and asexual reproduction — any time a body needs more cells that are exact copies.
- Mitosis
- The division of a nucleus into two genetically identical daughter nuclei, each with the same number of chromosomes as the parent.
- Chromosome
- A length of DNA carrying genes. Before mitosis it has been copied, so it is made of two identical sister chromatids joined at a centromere.
- Sister chromatids
- The two identical copies of a chromosome, produced by DNA replication and joined together at the centromere until anaphase.
- Centromere
- The point where the two sister chromatids are joined; spindle fibres attach here.
- Diploid
- Having the full (two of each) set of chromosomes. Mitosis keeps daughter cells diploid — the chromosome number does not change.
- Spindle
- A framework of protein fibres that attaches to chromosomes and pulls them to opposite ends of the cell.
Copied first, then shared out: Before mitosis even starts, the cell copies all of its DNA (this happens in interphase, the resting stage before division).
So each chromosome arrives at mitosis as two identical sister chromatids. Mitosis then simply shares one copy out to each daughter — which is exactly why the two daughters end up identical.
Mitosis runs through four phases, always in the same order: prophase, metaphase, anaphase, telophase.
A common way to remember the order is PMAT — or the phrase People Meet And Talk.
The four phases — PMAT: Prophase — chromosomes condense (coil up) and become visible; the spindle starts to form and the nuclear membrane breaks down.
Metaphase — chromosomes line up single file along the middle of the cell.
Anaphase — sister chromatids are pulled apart to opposite poles.
Telophase — two new nuclear membranes form, giving two nuclei.
| Phase (in order) | What the chromosomes do | Memory hook |
|---|---|---|
| Prophase | Chromosomes condense (coil up) and become visible; each is two identical sister chromatids joined at a centromere. The spindle begins to form and the nuclear membrane breaks down. | Prophase = Prepare / Appear |
| Metaphase | Chromosomes line up single file along the middle (equator) of the cell, attached to spindle fibres. | Metaphase = Middle |
| Anaphase | The centromeres split and the sister chromatids are pulled Apart to opposite poles by the spindle fibres. | Anaphase = Apart |
| Telophase | Two new nuclear membranes form around the two groups of chromosomes, which then uncoil. Two nuclei now exist. | Telophase = Two nuclei |
Before anaphase
- Each chromosome = two identical sister chromatids
- Chromatids are joined at the centromere
- Prophase: chromosomes condense and appear
- Metaphase: chromosomes line up at the middle
Anaphase onwards
- Centromeres split
- Sister chromatids are pulled apart
- One copy of every chromosome goes to each pole
- Telophase: two identical nuclei form
Why the two daughters are genetically identical: This is the headline exam point for this topic. Two facts working together guarantee it:
1) The DNA was replicated once beforehand, so the two sister chromatids of each chromosome are exact copies (same genes).
2) In anaphase those chromatids separate, with one copy going to each pole.
The result: each daughter receives one complete, identical set of chromosomes — same genes, same chromosome number (still diploid).
| Step in mitosis | What it guarantees | Why this keeps the daughters identical |
|---|---|---|
| DNA is replicated once before mitosis (in interphase) | Each chromosome becomes two identical sister chromatids | The two copies carry exactly the same genes |
| Sister chromatids stay joined until anaphase | The identical copies are kept paired up | One full copy is ready for each daughter cell |
| In anaphase, sister chromatids separate — one to each pole | Each pole receives one of every chromosome | Each daughter gets the same complete set of chromosomes |
| Two nuclei form (telophase), then the cell divides | Two cells, each with a full diploid set | Both daughters have the same genes AND the same chromosome number as the parent |
A memory hook for the phases: Prophase = Prepare (chromosomes appear). Metaphase = Middle (they line up). Anaphase = Apart (chromatids separate). Telophase = Two nuclei.
PMAT — People Meet And Talk.
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How this is tested: The single most common format is a Paper 1A / 1B micrograph of dividing cells (usually an onion root-tip or a whitefish cell): you must identify the mitotic phase of a labelled cell — read the chromosome behaviour (lined up = metaphase, being pulled apart = anaphase).
Another frequent Paper-1A item asks you to identify an event that occurs in BOTH mitosis and meiosis — DNA replication, chromosome condensation and spindle formation all happen in both.
The headline Paper 2 question is an Outline (3-4 marks): explain how chromosome behaviour during mitosis produces genetically identical daughter cells. Score it by linking DNA replication → identical chromatids → chromatids separate → one copy per cell.
IB-style question — outline how mitosis produces identical cells
Outline how the behaviour of chromosomes during mitosis produces two genetically identical daughter cells. [4]
How to score all four marks
- Start with replication. Before mitosis, the DNA is replicated, so each chromosome becomes two identical sister chromatids joined at the centromere.
- Line up. In metaphase the chromosomes line up at the equator (middle) of the cell, attached to spindle fibres.
- Separate the copies. In anaphase the centromeres split and the sister chromatids are pulled to opposite poles — so one identical copy of every chromosome moves to each end.
- Two identical nuclei. Each pole therefore receives a complete, identical set of chromosomes, and two genetically identical diploid nuclei form. (Award 1 mark per distinct point, up to 4.)
Final answer
DNA is replicated, so each chromosome is two identical sister chromatids; chromosomes line up in the middle; in anaphase the chromatids separate, one copy to each pole; so each daughter nucleus gets an identical, complete set of chromosomes.
✓ Why this scores full marks: Each sentence is a separate, distinct step in the chain: replication → identical chromatids → separate in anaphase → identical sets.
An Outline worth 4 marks needs four scoring points — the marker is looking for the cause-and-effect chain, not 'the cells are just copied' said four ways.
| Step in mitosis | What it guarantees | Why this keeps the daughters identical |
|---|---|---|
| DNA is replicated once before mitosis (in interphase) | Each chromosome becomes two identical sister chromatids | The two copies carry exactly the same genes |
| Sister chromatids stay joined until anaphase | The identical copies are kept paired up | One full copy is ready for each daughter cell |
| In anaphase, sister chromatids separate — one to each pole | Each pole receives one of every chromosome | Each daughter gets the same complete set of chromosomes |
| Two nuclei form (telophase), then the cell divides | Two cells, each with a full diploid set | Both daughters have the same genes AND the same chromosome number as the parent |