Watch a single cell condense its chromosomes, line them up at the equator, pull sister chromatids apart, and split into two genetically identical daughter cells — the engine of growth, repair, and asexual reproduction.
The model uses 2n = 4 so you can follow every chromosome; human body cells have 2n = 46. For the whole cycle (G1, S, G2 and checkpoints) see The Cell Cycle in 3D.
⬇ Further down: the original 2D simulation with the cell-cycle timeline, DNA-content graph and mitosis-vs-meiosis comparison.
Each diagram shows an animal cell with 2n = 4 chromosomes so you can follow every one. The numbers point to the parts named under the diagram. Tap through the stages in order, then test yourself below.
This is the diagram you are asked to label in tests. Switch between the labeled diagram, a blank one, and a quiz where you place each label yourself. Download both versions or print a label worksheet.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back. On a phone, swipe the diagram sideways to see every number.
Tap the diagrams in the order they happen (interphase first).
The tip of an onion root grows fast, so many of its cells are dividing. This is a stylised view through a microscope of stained root tip cells (plant cells: a cell wall and, in telophase, a cell plate instead of a cleavage furrow). Classify every cell, then work out the mitotic index and how long each stage lasts.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, replacing cells, asexual reproduction | Making gametes (sperm and egg cells; spores in plants and fungi) |
| Where (in humans) | Body (somatic) cells | Testes and ovaries |
| Number of divisions | 1 | 2 (meiosis I and meiosis II) |
| Daughter cells | 2 | 4 |
| Chromosome number | Same as the parent: diploid (2n). Humans 46 → 46 | Halved: haploid (n). Humans 46 → 23 |
| Genetically | Identical to the parent and to each other | Different from the parent and from each other |
| Homologous chromosomes pair up | No | Yes, in prophase I |
| Crossing over | No | Yes, in prophase I |
| Line up at metaphase | Single chromosomes in one row | Homologous pairs (metaphase I), then single chromosomes (metaphase II) |
| What separates in anaphase | Sister chromatids | Homologous chromosomes (anaphase I), then sister chromatids (anaphase II) |
Tap a statement, then tap the column it belongs in. Tap a placed statement to take it back.
Want every stage of meiosis? Open the meiosis page →
Mitosis = nuclear division. The full M phase = mitosis (karyokinesis) + cytokinesis. Each daughter cell receives a genetically identical full set of chromosomes — that's why mitosis is the basis of growth, repair, and asexual reproduction.
Scrape your knee and a week later the skin is whole again. Where did the new skin come from? Each cell that was lost is replaced when a neighbouring cell makes a perfect copy of itself and splits in two. That copy-and-split is mitosis. It is how one fertilised egg becomes a whole baby, how a cut heals, and how a strawberry plant sends out runners that grow into new plants. The golden rule is simple: one cell in, two identical cells out — same instructions, no mistakes.
To copy itself cleanly, a cell can't just rip its instructions in half — that would leave each daughter with only part of the manual. So it does the sensible thing first: it makes a full second copy. The instructions live on chromosomes, and before dividing the cell duplicates every one. The two matching copies, called sister chromatids, stay clipped together at a point called the centromere (that's the familiar X-shape). A human cell carries $2n = 46$ chromosomes; after copying it temporarily holds 92 chromatids, then hands a clean set of 46 to each daughter. Copy first, then split — that's the whole trick.
Biologists track the amount of DNA with the symbol $c$, where $1c$ is the DNA in a single (haploid) set. A resting cell sits at $2c$. During S phase the DNA doubles to $4c$. Mitosis then divides that $4c$ evenly, so each daughter ends back at $2c$ — full, not half. The visible division marches through four phases you can scrub through above: Prophase (chromosomes condense), Metaphase (they line up at the equator), Anaphase (sister chromatids are pulled to opposite poles), and Telophase (two new nuclei form), followed by cytokinesis splitting the cell. How often a tissue is caught mid-division is captured by the mitotic index, $\text{MI} = T_M / T_{\text{cycle}}$. For a 24-hour cycle with mitosis lasting about 1 hour, $\text{MI} \approx 1/24 \approx 4\%$ — which is why, at any instant, most cells you look at are not dividing.
Try this in the sim above. Step slowly from Metaphase into Anaphase and watch the moment the joined chromatids let go and slide apart — that single instant is what separates the two phases. Next, switch the cell type to Colchicine-treated: the spindle never forms, so the chromosomes are stranded and the cell stalls — the same trick doctors use to freeze chromosomes for a karyotype. Finally, open the DNA Content Graph tab and follow the line climb from $2c$ to $4c$ during S phase and drop back to $2c$ at cytokinesis — proof that the cell doubles before it halves.
Mitosis — Walther Flemming (1882) first described and named the process from observations of salamander cells; Theodor Boveri & Walter Sutton (1902–1904) showed chromosomes carry hereditary information; Tim Hunt, Lee Hartwell & Paul Nurse (Nobel Prize 2001) discovered the cyclin/CDK control system. Mitosis is the process by which a single eukaryotic cell divides its replicated chromosomes equally into two genetically identical daughter cells. It is the engine of growth (a baby becomes an adult through ~10¹⁴ mitotic divisions), tissue repair (skin, gut lining, blood cells), and asexual reproduction in many organisms.
where n = number of chromosome types and c = DNA content multiple (1c = haploid DNA quantity).
| Symbol / Structure | Biological meaning | When visible | A-level relevance |
|---|---|---|---|
| Chromatin | Loose DNA + histone proteins | Interphase | Not visible as discrete chromosomes |
| Chromosome | Condensed chromatin, visible under microscope | Prophase onwards | Made of 2 sister chromatids after S phase |
| Sister chromatids | Two identical DNA molecules joined at centromere | Prophase, metaphase | Result of S-phase replication |
| Centromere | Constricted region joining sister chromatids | All phases | Spindle attachment point |
| Kinetochore | Protein complex on each centromere | Prophase–anaphase | Where spindle microtubules attach |
| Centrosome / Centriole pair | Microtubule-organising centre | Always (one per cell, doubles during S) | Sends out spindle fibres |
| Spindle fibres | Microtubules pulling chromosomes | Prophase–anaphase | Made of tubulin protein |
| Metaphase plate | Imaginary plane at cell equator | Metaphase | Where all chromosomes align |
| $2n$ | Diploid chromosome number | — | Humans 2n = 46; fruit fly 2n = 8 |
| $c$ | DNA content (1c = haploid amount) | — | 2c → 4c during S; 4c → 2c in anaphase/telophase |
| MI | Mitotic index = (cells in mitosis / total) × 100% | — | Measure of how actively a tissue divides |
Why does the cell need mitosis? Multi-cellular life is impossible without it. A human starts as one fertilised egg and grows to ~30 trillion cells — every one a descendant of that zygote through mitotic divisions. Every minute, ~300 million of your cells divide to replace lost ones (skin, gut, blood, immune cells). Mitosis must be precise: every daughter must get exactly the right chromosomes.
The cell cycle follows a strict timing. For a typical human somatic cell with a 24-hour cycle:
Typical values: $T_{G_1} \approx 11$ h, $T_S \approx 8$ h, $T_{G_2} \approx 4$ h, $T_M \approx 1$ h.
A standard A-level technique is counting cells in a stained tissue under a microscope and calculating the mitotic index — the fraction of cells caught in the act of dividing:
For a continuously dividing tissue, the mitotic index is approximately the fraction of cycle time spent in mitosis:
For human gut lining (cycle ~24 h, mitosis ~1 h), MI ≈ 4%. For brain neurons (do not divide), MI ≈ 0%. For cancer cells, MI can exceed 20% — making it a key diagnostic indicator of tumour aggressiveness.
For a cell of diploid number $2n$ (humans: $2n = 46$):
| Phase | Chromosomes | Chromatids | DNA content |
|---|---|---|---|
| G1 | 2n = 46 | 2n = 46 (1 per chrom.) | 2c |
| After S phase / G2 / Prophase / Metaphase | 2n = 46 | 4n = 92 (2 per chrom.) | 4c |
| Anaphase (separation moment) | 4n = 92 (each chromatid is now a chromosome) | — | 4c |
| Each daughter after telophase + cytokinesis | 2n = 46 | 2n = 46 | 2c |
The default Cell View tab is a top-down view of a single dividing cell. The cell membrane is the outer ring; the nuclear envelope (when present) is the inner ring. Chromosomes are drawn as paired bars — two sister chromatids joined at an orange centromere. Homologous pairs are colour-coded (blue, red, yellow, purple) so you can track which chromatid goes where. The spindle fibres (purple) emerge from two centrosomes (orange) at opposite poles. Watch the chromosomes condense in prophase, line up in metaphase, separate in anaphase, and the cell pinch in two during cytokinesis.
The Cell Cycle Timeline tab shows the proportional durations of G1, S, G2, and M phases. The DNA Content Graph plots how the DNA amount per cell changes through the cycle — flat at 2c during G1, doubling during S to 4c, flat at 4c through G2 and M, then dropping back to 2c at cytokinesis. The Mitosis vs Meiosis tab side-by-side compares the two divisions to emphasise their fundamental differences.
Try the Colchicine-treated preset — colchicine is a real drug that binds tubulin and prevents spindle formation. The cell gets stuck in metaphase with chromosomes free-floating, unable to separate. This is exploited in karyotyping (so you can photograph chromosomes at maximum condensation) and was the basis of early chemotherapy.
A student stains an onion root tip and counts cells in 5 microscope fields. The results are:
| Field | Total cells | Prophase | Metaphase | Anaphase | Telophase |
|---|---|---|---|---|---|
| 1 | 52 | 3 | 1 | 0 | 1 |
| 2 | 48 | 2 | 2 | 1 | 0 |
| 3 | 55 | 4 | 1 | 0 | 2 |
| 4 | 50 | 3 | 2 | 1 | 1 |
| 5 | 45 | 2 | 1 | 0 | 1 |
| Total | 250 | 14 | 7 | 2 | 5 |
Q1: Calculate the mitotic index.
Cells in mitosis = 14 + 7 + 2 + 5 = 28
$$ \text{MI} = \dfrac{28}{250} \times 100\% = 11.2\% $$
Q2: If total cycle time is 20 hours, estimate the duration of M phase.
$$ T_M = \text{MI} \times T_{\text{cycle}} = 0.112 \times 20\ \text{h} = \mathbf{2.24\ h} $$
Q3: Why is prophase the most-counted phase?
Because prophase lasts longer than the other mitotic phases — chromosome condensation and spindle assembly take time, while anaphase (mechanical separation) is over in minutes. A higher mitotic-index reading for one phase reflects that phase's duration, not its importance. Takeaway: the mitotic index reads time, not significance — and is the standard A-level technique for measuring how actively a tissue is dividing.
❌ Misconception: "Sister chromatids separate in metaphase."
✅ Correction: Sister chromatids separate in anaphase, not metaphase. In metaphase, they are aligned at the equator, still held together at the centromere by cohesin protein. Only at the metaphase-to-anaphase transition does the enzyme separase cleave cohesin, and the spindle fibres then pull the now-separated chromatids to opposite poles. Mixing up metaphase (alignment) with anaphase (separation) is the most common diagram-labelling error in exams.
📖 Campbell & Reece, Biology, 12th ed., Ch. 12, Figure 12.7; Alberts, Ch. 17.5.
❌ Misconception: "Each daughter cell after mitosis has half the DNA of the parent."
✅ Correction: Each daughter cell has the same amount of DNA as the original G1 parent (2c) — and genetically identical to it. The confusion arises because students forget that DNA was doubled during S phase before mitosis. The parent cell at the start of mitosis had 4c (doubled); mitosis splits 4c into two 2c daughters. Saying "halved" is correct if you mean "halved from the 4c at metaphase", but wrong if you compare daughter to G1 parent — they are equal.
📖 Jones et al., Cambridge International AS & A Level Biology, 5th ed., Ch. 5.
❌ Misconception: "Mitosis only happens during growth."
✅ Correction: Mitosis happens throughout your entire life — to replace lost cells. Your gut lining is completely renewed every 5–7 days; red blood cells live ~120 days and are replaced at ~2 million per second; skin cells slough off and are replaced constantly. Even adult tissues like liver retain the capacity for mitosis after injury. The only major exceptions are neurons (mostly post-mitotic in adults) and cardiac muscle cells (very limited turnover). For most of your body, mitosis is a continuous, life-long process — not just for growing up.
📖 Alberts et al., Molecular Biology of the Cell, 7th ed., Ch. 22 "Stem Cells and Tissue Renewal".
❌ Error: Drawing chromosomes as X-shapes during interphase.
✅ Correct approach: During G1 interphase, each chromosome is a single DNA molecule — drawn as one bar, not an X. The X-shape only appears after S phase, when DNA has replicated to give two sister chromatids joined at a centromere. So G1 = single bars; G2 + prophase + metaphase = X-shapes; after anaphase separation = single bars again. Examiners deduct marks when students draw chromosomes consistently as X-shapes regardless of phase.
🔍 Textbook diagrams show condensed chromosomes (which are X-shaped post-S phase) so students assume that's the universal shape.
❌ Error: Computing mitotic index without recognising that prophase is over-represented because it lasts longer.
✅ Correct approach: The fraction of cells in each phase is proportional to the phase's duration, not its biological importance. If you see 4× more prophase cells than anaphase cells, prophase lasts ~4× longer than anaphase — that's the standard interpretation. To estimate the duration of any phase: $T_{\text{phase}} = \dfrac{\text{cells in phase}}{\text{total cells in mitosis}} \times T_M$. This is a classic A-level practical calculation worth knowing.
🔍 Students confuse cell count with reaction rate — but in a steady-state population, count reflects time spent.
❌ Error: Writing "mitosis produces gametes."
✅ Correct approach: Mitosis produces somatic (body) cells — identical, diploid daughter cells used for growth and repair. Gametes (sperm and egg) are produced by meiosis in the gonads, which halves the chromosome number and produces four genetically unique haploid cells. Confusing mitosis with meiosis is the single most common exam mistake in cell-division questions. Always check: identical or different? diploid or haploid? somatic or gamete?
🔍 Both processes start similarly and use the same terminology (prophase, metaphase, etc.), making them easy to mix up.