Mitosis: Watch Every Stage of Cell Division

How to use this simulation

1. Press ▶ and the cell runs through the whole cycle on its own: interphase, prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis.
2. Use “Previous stage” and “Next stage” to move one stage at a time; the animation pauses each time so you can look properly.
3. Drag the timeline slider to any moment. The text under the cell says what is happening at that instant.
4. Switch the chromosome number from 2n = 4 to 6: a dark and a light chromosome of the same colour are one homologous pair.
5. Change the cell type to a plant cell: there are no centrioles, and at the end a cell plate forms in the middle instead of a furrow.
6. Copy the chromosome, chromatid and DNA readings for each phase into your notebook and check them against the table further down.

From one cell to trillions: how?

You started life as a single cell, a zygote. Today your body holds tens of trillions of cells. Where did they all come from? The answer is simple: one cell splits into two, two into four, four into eight. That splitting is cell division.

Cut your finger and within days the wound closes. Who makes the new skin? The cells around the wound divide again and again until the gap is filled. Hair grows, nails lengthen, a plant root pushes deeper into the soil: the same job is going on everywhere.

There is one big condition, though. A new cell has to receive exactly the same information as its parent. A skin cell must give skin cells, and each one must carry the right number of chromosomes. The process that shares everything out this precisely is mitosis. The cell first copies all of its DNA, then sends one complete copy to each side.

Textbooks show this as a still picture that we memorise. But mitosis is really a dance: the chromosomes line up in pairs in the middle, then suddenly rush apart to opposite ends. The animation above shows that dance. Let us take it step by step.

What is cell division, and what are its types?

Cell division is the process by which one living cell divides to form two or more daughter cells. The dividing cell is called the parent (mother) cell, and the new ones are the daughter cells.

In 1855 Rudolf Virchow argued that every cell comes from a pre-existing cell. In 1882 the German biologist Walther Flemming, watching dividing salamander cells, named the process mitosis, from the Greek mitos, meaning thread, because the chromosomes look like threads when division begins.

Three kinds of division are seen in living cells. This is the answer to the classic question “how many types of cell division are there?”:

• Amitosis (direct division): the nucleus simply pinches in the middle into two, then the cytoplasm splits. No spindle forms and no distinct chromosomes appear. It is seen in simple single-celled organisms such as amoeba and yeast.
• Mitosis (equational division): the nucleus divides once and the daughter cells keep the same chromosome number as the parent. It happens in body (somatic) cells.
• Meiosis (reduction division): the nucleus divides twice but the chromosomes are copied only once, giving four daughter cells, each with half the chromosome number. It happens in reproductive mother cells when gametes are made.

The cell cycle: the long preparation before division

The cell cycle is the sequence of events from the moment a cell is formed until it divides into two. It has two main parts: interphase and the M phase (mitotic phase). Surprisingly, a cell spends most of its life in interphase. In many human cells in culture, the whole cycle takes about 24 hours, while the actual division lasts only about an hour.

Interphase used to be called the “resting phase”, because no chromosomes are visible under the microscope. It is anything but restful: like the night before an exam, this is when the cell is busiest. Interphase has three sub-phases:

The stages of mitosis in detail

Mitosis is studied in two parts. First the nucleus divides, which is called karyokinesis; then the cytoplasm divides, called cytokinesis. Karyokinesis has five stages: prophase, prometaphase, metaphase, anaphase and telophase (many books fold prometaphase into prophase and list four). The stages actually flow into one another; we separate them only to make them easier to learn. Pause the animation at each stage as you read about it.

How to count chromosomes and chromatids

Most mistakes in mitosis problems are counting mistakes. There is one rule: count chromosomes by counting centromeres. As many centromeres, as many chromosomes. A chromosome with two chromatids is still one chromosome, because it has one centromere.

The table below uses the animation's own setting, 2n = 4. Run the animation and check the readings panel against it.

The same count for a human cell (2n = 46)

A human body cell has 46 chromosomes, that is 23 pairs. The same rules give the count at every phase below. A favourite exam question is “how many chromatids are there in a human cell at metaphase?” The answer is 92, yet the chromosome number is still 46.

Mitosis in plant cells vs animal cells

The main stages are the same, but a plant cell’s rigid wall and its lack of centrioles cause a few differences. Switch the cell type in the animation to see them.

Mitosis vs meiosis

This comparison comes up in almost every school biology exam. Understand the table and you will not need to memorise it.

Why mitosis matters

Without mitosis there would be no multicellular life at all. These are the points to write for “state the significance of mitosis”:

• Growth: a whole body develops from a zygote through repeated mitosis. A plant grows longer and thicker through mitosis in its meristems.
• Repair and healing: millions of skin, gut-lining and blood cells die every day, and mitosis replaces them.
• A constant chromosome number: every body cell carries the same chromosomes and genes, so every cell follows the same instructions.
• Asexual and vegetative reproduction: from amoeba and yeast to cuttings, grafting and plant tissue culture, all depend on mitosis.
• Balance between nucleus and cytoplasm: a cell that grows too large cannot be controlled well by one nucleus; dividing restores the balance.

Try this in the simulation

Seeing it yourself sticks far better than reading. Try these five short experiments:

• Pause in S phase: a faint copy appears beside every thread. The chromosome reading stays the same while the chromatid reading doubles.
• Compare the start and end of prophase: the same chromosome turns from a long thread into a short thick rod, and the nuclear envelope turns dashed as it starts to break up.
• Pause at metaphase and count the black centromere dots. With 2n = 6 you will find six dots and twelve chromatids.
• Pause right at the start of anaphase, then step forward: the moment the centromeres split, the chromosome reading doubles.
• Choose a plant cell and watch cytokinesis: no furrow, just a cell plate growing up and down from the middle.

Solved problems, step by step

Calculations on mitosis turn up in application and higher-order questions. Work through these in your own notebook.

Practical: observing mitosis in an onion root tip

This is the classic school and college practical. The tip of an onion root is a meristem, where cells divide all the time, so one slide can show every stage of mitosis.

• Rest an onion on the mouth of a glass of water for a few days until white roots grow.
• Cut off the last 1–2 mm of a root tip, where division is most active.
• Soak the tip briefly in dilute hydrochloric acid to loosen the cells.
• Stain with acetocarmine or aceto-orcein so the chromosomes take up a dark colour.
• Place it on a slide, add a cover slip and press gently (a squash), then look under the microscope.
• Find cells in interphase, prophase, metaphase, anaphase and telophase, draw them, and count them to work out the mitotic index.

Mistakes almost everyone makes

Avoid these and full marks on mitosis questions become easy.

• Saying chromosomes double in S phase. The DNA and the chromatids double; there is still one centromere, so the chromosome number stays the same.
• Writing 4n chromosomes at metaphase. At metaphase there are 2n chromosomes and 4n chromatids; the chromosome number doubles at anaphase.
• Thinking “resting phase” means nothing happens in interphase. It is the busiest phase, and DNA replication happens in it.
• Saying mitosis happens only in diploid cells. Some haploid cells also divide by mitosis.
• Drawing metaphase chromosomes scattered about. The centromeres must lie on one straight line in the middle.
• Showing a furrow in a plant cell’s cytokinesis. Plants form a cell plate.

Mitosis in real life

Cancer is mitosis out of control. A normal cell knows when to stop, but when certain genes are damaged the cell ignores the stop signals and keeps dividing, forming a tumour. Many cancer drugs therefore block spindle formation or DNA replication to stop rapidly dividing cells, which is also why chemotherapy makes hair fall out: hair-root cells divide quickly too.

A chemical called colchicine prevents the spindle from forming, so cells get stuck at metaphase. Scientists use this to photograph metaphase chromosomes and build a karyotype, which can reveal chromosomal conditions such as Down syndrome.

In plant tissue culture, thousands of identical plantlets are grown from one small piece of plant, because cells made by mitosis are genetically identical. Banana, potato and orchid plantlets are produced this way. Stem-cell research and growing skin for burn patients also rely on mitosis.

Exam corner

In school exams a mitosis question usually gives a diagram and asks which stage it shows, what happens in that stage, and why mitosis matters. At higher-secondary level (Class 11, A level, AP Biology) the questions go deeper: the cell cycle, the comparison with meiosis, and chromosome and DNA counts.

• Definitions: mitosis, karyokinesis, cytokinesis, kinetochore, cell cycle, mitotic index.
• Reasoning: why is mitosis called equational division? Why is metaphase best for counting chromosomes? Why is “resting phase” a poor name for interphase?
• Application: find the chromosome and chromatid numbers in a pictured cell; find the number of cells after a given number of divisions.
• Analysis: what would happen to an organism without mitosis? How do plant and animal cell division differ, and why?
• In diagrams of metaphase and anaphase, always label the spindle fibres, centromeres and poles.

Revision summary

Read only this section the night before the exam and the key points will come back.

• Mitosis = equational division: 1 parent cell → 2 identical daughter cells, chromosome number unchanged.
• Cell cycle = interphase (G₁ → S → G₂) + M phase; DNA replication happens in S phase.
• Karyokinesis: prophase → prometaphase → metaphase → anaphase → telophase; then cytokinesis.
• Prophase is the longest stage and anaphase the shortest; metaphase is the best time to count chromosomes.
• Count chromosomes by centromeres; at anaphase the whole cell holds twice the chromosome number.
• Animal cells divide by a furrow, plant cells by a cell plate; animal cells have centrioles and asters.
• Significance: growth, repair, asexual reproduction and a constant chromosome number; uncontrolled mitosis causes cancer.

Frequently asked questions