What you'll learn
Mitosis is the form of cell division that produces two genetically identical cells from one. This topic covers the cell cycle and its stages, what happens to the chromosomes during mitosis, why the products are identical, where mitosis occurs in the body, and how it differs from meiosis. It also covers cell differentiation, stem cells and their medical uses, and the therapeutic cloning debate.
Examiners test two things repeatedly here: whether you can put the stages of the cell cycle in the right order with the right event attached to each, and whether you can keep mitosis and meiosis apart. Confusing the two is the most costly single error in this part of the specification, because it makes every subsequent answer wrong.
Key terms and definitions
Mitosis — cell division producing two genetically identical diploid daughter cells.
Meiosis — cell division producing four genetically different haploid gametes.
Chromosome — a long molecule of DNA carrying many genes.
Chromatid — one of the two identical halves of a replicated chromosome.
Diploid — having two sets of chromosomes; in humans, 46.
Haploid — having one set of chromosomes; in humans, 23.
Cell cycle — the sequence of growth and division a cell passes through.
Differentiation — the process by which a cell becomes specialised for a particular function.
Stem cell — an undifferentiated cell able to divide and give rise to specialised cells.
Embryonic stem cell — a stem cell from an early embryo, able to form any cell type.
Adult stem cell — a stem cell in mature tissue, able to form a limited range of cell types.
Meristem — a region in a plant where cells remain able to divide throughout life.
Core concepts
The cell cycle
Before a cell can divide it must grow and copy its contents. The cell cycle has three stages, and the order matters.
Growth and DNA replication. The cell increases in size, the number of sub-cellular structures such as mitochondria and ribosomes increases, and the DNA replicates so that each chromosome becomes two identical chromatids joined together. This is the longest stage of the cycle by far.
Mitosis. One set of chromosomes is pulled to each end of the cell, and the nucleus divides. The chromatids are separated so that each new nucleus receives one copy of every chromosome.
Cytokinesis. The cytoplasm and cell membrane divide, producing two identical daughter cells.
A frequent examination question asks why the DNA must replicate before mitosis rather than after. The answer is that each daughter cell must receive a complete copy of every chromosome, and that is only possible if a second copy already exists to be separated.
What happens to the chromosomes
Humans have 46 chromosomes in each body cell, arranged as 23 pairs. During the growth stage each of the 46 chromosomes is copied, giving 46 pairs of chromatids. In mitosis these pairs line up along the centre of the cell, the chromatids are pulled apart to opposite poles, and two nuclei form, each containing 46 chromosomes.
Both daughter cells therefore have the same number of chromosomes as the parent — mitosis does not halve the chromosome number — and the same genetic information. The cells are clones of the parent cell and of each other.
Where mitosis happens and why it matters
Mitosis is used wherever new identical cells are needed:
- Growth of a multicellular organism from a fertilised egg
- Repair of damaged tissue, such as healing a wound
- Replacement of cells that wear out, such as those lining the gut or the skin
- Asexual reproduction in some organisms, producing offspring genetically identical to the parent
In plants, mitosis is concentrated in regions called meristems, at root and shoot tips, which is why plants continue growing at those points throughout life.
Because mitosis produces identical cells, it cannot generate variation. Variation in sexually reproducing species comes from meiosis and from fertilisation, not from mitosis.
Observing mitosis under the microscope
Mitosis is usually observed in a root tip, because that is a meristem where many cells are dividing at any moment. The tip is treated with acid to separate the cells, stained so the chromosomes become visible, and gently squashed under a cover slip to give a layer thin enough for light to pass through.
Under the microscope most cells appear not to be dividing, because the growth stage occupies far the largest part of the cell cycle. The proportion of cells visibly in mitosis therefore estimates how long mitosis takes relative to the whole cycle — a calculation examiners sometimes set. If 6 cells out of 60 are in mitosis, mitosis occupies about a tenth of the cycle.
Mitosis compared with meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | one | two |
| Daughter cells produced | two | four |
| Chromosome number | unchanged, diploid | halved, haploid |
| Genetic result | identical to parent | genetically different |
| Occurs in | body cells | reproductive organs |
| Purpose | growth, repair, replacement | producing gametes |
Two of these lines carry most of the marks. Mitosis gives two identical diploid cells; meiosis gives four different haploid cells. If you can state that reliably, most comparison questions resolve themselves.
Uncontrolled division and cancer
Cell division is normally tightly controlled. If that control fails, cells divide repeatedly and uncontrollably, forming a tumour. A benign tumour stays in one place; a malignant tumour invades surrounding tissue and cells may spread in the blood to form secondary tumours elsewhere. Cancer is therefore best described in an examination as uncontrolled cell division rather than as a single disease.
Differentiation and stem cells
Most cells in an adult animal are differentiated: they have developed a structure suited to one job and can no longer become a different type. In animals the ability to differentiate is largely lost early in development, and most later division is for repair and replacement. In plants, by contrast, many cells retain the ability to differentiate throughout the plant's life, which is why a cutting can grow into a whole new plant.
Stem cells are undifferentiated and can divide to produce cells that then specialise.
- Embryonic stem cells, from early embryos, can differentiate into any cell type.
- Adult stem cells, found in tissues such as bone marrow, form a limited range — bone marrow stem cells produce blood cells.
- Plant meristem cells can produce any plant cell type and can be used to clone plants quickly and cheaply.
Using stem cells in medicine
Stem cells offer treatments for conditions where cells have been lost or no longer work, such as paralysis from spinal injury, type 1 diabetes, and some blood disorders treated by bone marrow transplant.
Therapeutic cloning produces an embryo with the same genes as the patient, so the stem cells taken from it are not rejected by the patient's immune system. The advantages are clear, but there are objections and risks. Some people object on ethical grounds to creating and destroying embryos. There is a risk of transferring viral infection with the donated cells, and in some cases a risk of the transplanted cells dividing uncontrollably. Answers that acknowledge both the benefit and a specific objection score better than one-sided ones.
Plant cloning from meristems is used to preserve rare species and to produce large numbers of identical crop plants with desirable characteristics — a commercial application worth knowing as a contrast to the animal case.
Worked examples
Example 1. A human body cell contains 46 chromosomes. State the number of chromosomes in each of the two cells produced by mitosis, and explain your answer.
Each daughter cell contains 46 chromosomes. Before division, every chromosome is copied to form two identical chromatids. During mitosis the chromatids are separated so that each new nucleus receives one copy of all 46, leaving the chromosome number unchanged and both cells genetically identical to the parent.
Example 2. Explain why cells in the lining of the small intestine divide by mitosis far more often than cells in the brain.
Cells lining the small intestine are exposed to abrasion and digestive enzymes and are worn away continually, so they must be replaced frequently, which requires repeated mitosis. Brain cells are not lost at anything like that rate and largely do not divide once differentiated, so mitosis there is rare.
Example 3. A student writes that mitosis produces variation within a species. Explain why this is incorrect and state what does produce variation.
Mitosis copies the DNA exactly and separates identical chromatids, so the daughter cells are genetically identical to the parent and no variation arises. Variation comes from meiosis, which produces genetically different gametes, and from the random combination of gametes at fertilisation, together with mutation.
Example 4. Give one advantage and one risk of using stem cells from a therapeutically cloned embryo to treat a patient.
An advantage is that the cells carry the patient's own genes, so they are not rejected by the immune system. A risk is that a virus present in the cultured cells could be transferred to the patient.
Common mistakes and how to avoid them
Saying mitosis halves the chromosome number. It does not — that is meiosis. Mitosis keeps the number the same and produces diploid cells.
Writing that mitosis produces four cells. Mitosis produces two; meiosis produces four.
Putting DNA replication after mitosis. Replication must come first, in the growth stage, or there would be nothing to separate.
Confusing chromosome with chromatid. A replicated chromosome consists of two identical chromatids joined together; the chromatids are what get separated.
Claiming embryonic and adult stem cells are equivalent. Embryonic stem cells can form any cell type; adult stem cells form a limited range. The distinction is usually worth a mark.
Describing cancer as a tumour. Cancer is the result of uncontrolled cell division; a tumour is the mass of cells that results.
Giving a one-sided answer on stem cell ethics. Questions asking you to evaluate expect both a benefit and an objection or risk.
Exam technique for Cell division by mitosis
When asked to describe the cell cycle, give the stages in order and attach the key event to each: growth with DNA replication, then mitosis with separation of chromatids, then cytokinesis dividing the cytoplasm. A list of stage names without events rarely gains full marks.
For comparison questions, write full comparative sentences covering both processes: "mitosis produces two identical diploid cells whereas meiosis produces four genetically different haploid cells". Stating only what mitosis does leaves the comparison incomplete.
Where a question gives a chromosome number, use it. If the cell has 12 chromosomes, say the daughter cells have 12 after mitosis and 6 after meiosis rather than answering in general terms.
For stem cell questions, read whether you are asked to describe a use or to evaluate it. An evaluation needs at least one point on each side and ideally a conclusion.
Quick revision summary
- The cell cycle: growth and DNA replication → mitosis → cytokinesis.
- DNA replicates before mitosis so each daughter cell receives a complete copy.
- A replicated chromosome consists of two identical chromatids, which mitosis separates.
- Mitosis gives two genetically identical diploid cells; meiosis gives four genetically different haploid cells.
- Mitosis is used for growth, repair, replacement and asexual reproduction, and produces no variation.
- In plants, mitosis is concentrated in meristems at root and shoot tips.
- Uncontrolled cell division forms a tumour; malignant tumours spread to form secondary tumours.
- Differentiated cells are specialised; most plant cells retain the ability to differentiate, most adult animal cells do not.
- Embryonic stem cells form any cell type; adult stem cells form a limited range.
- Therapeutic cloning avoids rejection because the cells share the patient's genes, but raises ethical objections and a risk of viral transfer.