What is mitosis?

You started as one cell. Every one of the tens of trillions of cells in your body today came from that first cell by mitosis, and each carries a copy of the same DNA. Right now, cells in your skin, gut lining and bone marrow are dividing by mitosis to replace the ones you lose every day.

What is the purpose of mitosis?

The purpose of mitosis is to make new cells that are exact genetic copies of the original. Organisms need that for four things:

• Growth: a baby grows into an adult by making more cells, not bigger ones.
• Repair: a cut heals and a broken bone mends as nearby cells divide to fill the gap.
• Replacement: skin cells, red blood cells and the cells lining your gut wear out and are replaced constantly.
• Asexual reproduction: single-celled organisms such as amoebas, and plants growing from cuttings or runners, use mitosis to make offspring identical to the parent.

Because the new cells are identical, they can do the same job as the cells they replace. A skin cell copied by mitosis is still a skin cell with exactly the same genes.

Mitosis and the cell cycle

Mitosis is one part of the cell cycle, the repeating sequence a cell goes through between one division and the next.

G1, S and G2 together are called interphase. These times are for a typical human cell dividing every 24 hours in a lab, from The Cell. Many cells divide far less often, and some, such as most nerve cells, stop dividing altogether. The key point: the DNA is copied in S phase, before mitosis begins, so each chromosome enters mitosis as two identical sister chromatids joined at the centromere.

The stages of mitosis

Mitosis is a continuous process, but biologists split it into stages to describe it. Many courses use four, remembered as PMAT: prophase, metaphase, anaphase and telophase. OpenStax and many university courses add a fifth, prometaphase, between prophase and metaphase.

1. Prophase

The long, thin chromosomes coil up and become short and thick enough to see under a light microscope, each as an X shape of two sister chromatids. The nucleolus disappears, and a spindle of protein fibres (microtubules) starts to form from the two ends, or poles, of the cell. Late in prophase, often called prometaphase, the nuclear envelope breaks down and spindle fibres attach to each chromosome at its centromere.

2. Metaphase

The spindle fibres move the chromosomes into a single line at the centre of the cell, along the metaphase plate (or equator). Each chromosome is attached to fibres from both poles. The cell checks that every chromosome is attached properly before going on.

3. Anaphase

The centromeres split and the spindle fibres shorten, pulling the sister chromatids apart to opposite poles. From this moment each chromatid counts as a chromosome in its own right. Anaphase is the shortest stage, and the chromatids look like V shapes as they are dragged centromere first.

4. Telophase

A full set of chromosomes arrives at each pole. A new nuclear envelope forms around each set, the chromosomes uncoil, and the nucleoli reappear. There are now two nuclei, each with the same chromosomes as the original.

Try naming the stage from a picture of a dividing cell, then press Mark to check your answer in our Cell Biology course.

Cytokinesis: what is the relationship between cytokinesis and mitosis?

Cytokinesis is the division of the cytoplasm that splits one cell into two. Mitosis divides the nucleus, and cytokinesis divides everything else. They are separate processes, but cytokinesis usually starts during late anaphase or telophase and finishes just after, so the two overlap. Mitosis can happen without cytokinesis, which is how some cells end up with more than one nucleus, like skeletal muscle fibres, which form when many cells fuse.

• In animal cells, a ring of protein filaments tightens around the middle of the cell like a drawstring, making a cleavage furrow that pinches the cell in two.
• In plant cells, the rigid cell wall can't pinch, so vesicles gather at the middle and fuse into a cell plate, which grows outwards into a new wall between the two cells.

Which of the following does not occur during mitosis?

Quiz questions often ask this, so it is worth knowing what is never part of mitosis.

• DNA replication. The DNA is copied in S phase of interphase, before mitosis starts.
• Pairing of homologous chromosomes and crossing over. These only happen in prophase I of meiosis.
• Halving the chromosome number. Mitosis keeps it the same. Only meiosis halves it.
• Making genetically different cells. Barring rare copying errors, the two daughter cells are identical.

Mitosis vs meiosis

Our guide to meiosis goes through its stages in the same detail.

Seeing mitosis: the root tip practical

Plant root tips are full of dividing cells, which makes them the classic way to see mitosis. In AQA's A-level Biology, preparing a stained squash of root tip cells and calculating a mitotic index is one of the required practicals. The method in short: cut the tip of a garlic or onion root, soften it in warm acid, stain the chromosomes, squash it thin under a cover slip, and look under a light microscope. You can practise spotting the stages and measuring cells in the Virtual Microscope.

The mitotic index is the percentage of cells in a sample that are in mitosis:

Worked examples

Example 1: mitotic index

A student counts 150 cells in a root tip. 6 are in prophase, 3 in metaphase, 2 in anaphase and 1 in telophase. What is the mitotic index? Cells in mitosis: 6 + 3 + 2 + 1 = 12. Mitotic index = 12 ÷ 150 × 100 = 8%.

Example 2: how long each stage lasts

If these cells divide every 24 hours, how long do they spend in mitosis, and in anaphase? The share of cells in a stage matches the share of time spent in it. Mitosis: 8% of 24 hours = 0.08 × 24 = 1.92 hours, about 115 minutes. Anaphase: 2 ÷ 150 × 24 × 60 = 19.2 minutes, the shortest stage, which is why so few cells are caught in it.

Example 3: chromosomes and DNA through the cycle

A human cell in G1 has 46 chromosomes and an amount of DNA x. After S phase it still has 46 chromosomes, but each has two chromatids, so the DNA is 2x. After mitosis and cytokinesis, each daughter cell has 46 chromosomes and DNA x, the same as the parent before it copied.

Example 4: doubling

A single cell divides by mitosis, and so does every new cell, ten times in a row. How many cells are there? Each division doubles the number, so 2¹⁰ = 1,024 cells. After 20 rounds there are 2²⁰, over a million. Doubling is why a tumour can grow so fast.

When mitosis goes wrong: cancer

The cell cycle has checkpoints that stop a cell dividing if its DNA is damaged or the chromosomes aren't attached properly. If mutations break these controls, a cell can divide again and again without stopping, forming a lump called a tumour. Benign tumours stay in one place. Malignant tumours, which are cancers, can invade nearby tissue and spread around the body. Many cancer drugs and radiotherapy work by damaging dividing cells, which is why they also affect fast-dividing healthy cells such as hair follicles.

Common mistakes

• Saying DNA is copied during mitosis. It is copied in S phase of interphase, beforehand.
• Counting interphase as a stage of mitosis. Interphase is part of the cell cycle, not of mitosis itself, though some courses write IPMAT to include it.
• Mixing up chromosomes and chromatids. After S phase, each chromosome is two chromatids. In anaphase the chromatids separate and each becomes a chromosome.
• Saying mitosis makes the cell divide. Mitosis divides the nucleus. Cytokinesis divides the cell.
• Saying mitosis halves the chromosome number. It keeps it the same. Meiosis halves it.

Read next

Biology · CellsThe cell cycle GCSE: the three stages explainedThe cell cycle for GCSE Biology: the three stages, what happens to DNA and organelles, why mitosis matters, exam answers, and what A-level adds.