The cell cycle
Cells do not divide at random — they follow the cell cycle. Most of the time a cell is in interphase, when it grows, carries out its normal work, and — crucially — copies its DNA so each new cell will get a full set. Only then does it divide, by mitosis or meiosis.
Interphase is the longest stage; division itself is brief by comparison.
Mitosis
Mitosis produces two genetically identical daughter cells, each with the same number of chromosomes as the parent (diploid). The copied chromosomes line up, and one copy of each is pulled to each end before the cell splits. Mitosis is the basis of growth, repair of damaged tissue, and asexual reproduction.
Because the cells are identical, mitosis produces clones — useful for a plant growing a new root, or your skin replacing worn cells.
Meiosis
Meiosis makes the gametes (sex cells — sperm, eggs, pollen) and does two special things. It halves the chromosome number, producing haploid cells, so that when two gametes fuse at fertilisation the normal diploid number is restored. And it shuffles the genes, so the four daughter cells are all genetically different — the source of variation in offspring.
| Mitosis | Meiosis | |
|---|---|---|
| Daughter cells | 2 | 4 |
| Chromosome number | same (diploid) | halved (haploid) |
| Genetically | identical | all different |
| Purpose | growth, repair, asexual reproduction | making gametes |
When division goes wrong: cancer
The cell cycle is tightly controlled. If that control fails, cells divide uncontrollably to form a tumour — the basis of cancer. Mutations caused by carcinogens such as tobacco smoke, ionising radiation or some chemicals can trigger this loss of control, which is why avoiding carcinogens lowers cancer risk.
Copy first, then divide
DNA is replicated during interphase, before division. This ensures each daughter cell in mitosis receives a complete, identical set of chromosomes.
A worked example
A human body cell has 46 chromosomes. State the number of chromosomes in a cell produced by (a) mitosis and (b) meiosis, and explain why meiosis must halve the number.
- (a) Mitosis keeps the number the same: 46 chromosomes.Mitosis produces genetically identical diploid cells.
- (b) Meiosis halves the number: 23 chromosomes.Gametes are haploid.
- At fertilisation a sperm (23) fuses with an egg (23).Two gametes combine.
- This restores 46 — so halving in meiosis keeps the number constant across generations.If gametes were diploid, the number would double each generation.
(a) 46 (mitosis); (b) 23 (meiosis). Halving in meiosis means fertilisation restores the normal diploid 46, keeping it constant.
Chromosomes, diploid and haploid
The DNA in a cell is organised into chromosomes. Body cells carry chromosomes in matching pairs and are diploid (humans have 23 pairs, 46 in all); gametes carry just one of each pair and are haploid (23). Before any division, each chromosome is copied during interphase, so it briefly consists of two identical chromatids joined together — ready to be separated when the cell divides.
Stem cells
Most cells are specialised and can no longer change their role, but stem cells are unspecialised cells that can keep dividing and can differentiate into different cell types. In animals they allow growth and the repair of tissues; in plants, regions called meristems act as stem cells throughout life. Their ability to become many cell types makes stem cells valuable in medicine — for example, to replace damaged tissue — though their use raises ethical questions.
Copied, then separated
Interphase copies each chromosome into two identical chromatids. Mitosis then separates the chromatids so each daughter cell gets one full, identical set — the reason mitosis produces genetically identical cells.