Why do cells divide? Cell theory
A cell gets food and oxygen through its outer skin, the cell membrane. When a cell grows, its inside grows faster than its skin. A very big cell cannot get enough food in, so it splits in two.
Cells divide for three reasons:
- Growth: a baby becomes an adult by making more cells, not bigger cells.
- Repair: new cells replace hurt or old cells (skin, blood).
- Reproduction: making a new living thing.
Cell theory has three simple ideas: (1) all living things are made of cells; (2) the cell is the basic unit of life; (3) every new cell comes from a cell that already existed. Scientists Schleiden, Schwann and Virchow built this idea in the 1800s.
Chromosomes: the instruction books
A chromosome is a long thread of DNA wrapped around proteins. DNA holds the instructions for the body. Chromosomes are in the nucleus.
Body cells have chromosomes in pairs: one of each pair came from the mother and one from the father. The two of a pair are called homologous chromosomes. The total number is written 2n. Humans have 2n = 46 (23 pairs).
Before a cell divides, each chromosome is copied. The two copies, called sister chromatids, stay joined at one point (the centromere), so the chromosome looks like an X.
| Living thing | Chromosomes in a body cell (2n) |
|---|---|
| Human | 46 |
| Dog | 78 |
| Rice | 24 |
| Fruit fly | 8 |
The cell cycle
The cell cycle is the life of a cell from one division to the next. It has two big parts:
- Interphase (most of the time): G1 – the cell grows and does its job; S – it copies (synthesises) its DNA; G2 – it makes what it needs for division.
- M phase (short): the nucleus divides (mitosis) and then the cytoplasm splits (cytokinesis).
A skin cell in a person may go round the cycle in about a day. Nerve cells mostly stop and do not divide again. Cells that divide when they should not can form a lump called a tumour; this is what happens in cancer.
Mitosis: two identical cells
Mitosis has four stages. A simple way to remember: PMAT.
- Prophase: chromosomes coil up and become thick and visible. The nucleus wall breaks down.
- Metaphase: chromosomes line up on the middle line (equator) of the cell. Thin fibres (spindle) hold each one.
- Anaphase: the sister chromatids of each chromosome are pulled to opposite ends.
- Telophase: a new nucleus forms at each end. Then the cell splits (cytokinesis). In animal cells the membrane pinches in; in plant cells a new wall (cell plate) forms in the middle.
Result: 2 daughter cells, identical to the parent, with the same number of chromosomes (2n → 2n). Mitosis is used for growth, repair and asexual reproduction.
Meiosis: four cells with half the chromosomes
Meiosis happens only in sex organs (testes, ovaries, anthers, ovules). It makes gametes (sperm, egg, pollen nuclei).
- Meiosis I: homologous chromosomes pair up. They may swap small pieces (crossing over). Then the pairs separate: each new cell gets one chromosome of each pair. The number halves here.
- Meiosis II: like mitosis, the sister chromatids separate.
Result: 4 cells, each with half the chromosomes (2n → n), and all different from each other. In humans the gametes have n = 23. When a sperm (23) joins an egg (23) at fertilisation, the baby's first cell has 46 again.
Why halve? If gametes had 46, the baby would have 92, and the number would double every generation.
Mitosis vs meiosis
| Mitosis | Meiosis | |
|---|---|---|
| Where | Body cells everywhere | Only sex organs |
| Number of divisions | 1 | 2 |
| Cells made | 2 | 4 |
| Chromosome number | Same (2n → 2n) | Half (2n → n) |
| Daughter cells | Identical to parent | Different from parent and each other |
| Use | Growth, repair | Making gametes; variety in children |
Try it: count like a biologist
At home: take 2 long and 2 short strips of paper in red, and the same in blue (2n = 4). Copy each strip (make an X with two strips). Now act out mitosis: line them up and pull the two halves of each X apart. Count: 2 cells, 4 strips each. Then act out meiosis: separate the red/blue partners first, then the halves. Count: 4 cells, 2 strips each.
In the 3D: at the last step, set 2n = 8 (like a fruit fly) and predict the result before you press Play.
Key formulas and definitions
- Mitosis: 1 cell (2n) → 2 cells (2n)
- Meiosis: 1 cell (2n) → 4 cells (n)
- Fertilisation: n + n = 2n
- After n rounds of mitosis: 1 cell → 2ⁿ cells
- Cell cycle: G1 → S → G2 → M (interphase = G1 + S + G2)
Worked examples
1. A dog body cell has 78 chromosomes. How many chromosomes are in (a) a skin cell made by mitosis, (b) a sperm cell?
(a) Mitosis keeps the number: 78. (b) Meiosis halves it: 78 ÷ 2 = 39.
2. One cell divides by mitosis every hour. How many cells are there after 5 hours?
Each division doubles the number: 1 → 2 → 4 → 8 → 16 → 32. So 2⁵ = 32 cells.
3. A cell with 2n = 4 goes through meiosis. How many cells are made and how many chromosomes does each have?
Meiosis gives 4 cells. Each has n = 4 ÷ 2 = 2 chromosomes, one from each pair.
4. A cell spends 24 hours in one cycle: G1 = 10 h, S = 8 h, G2 = 4 h. How long is M phase, and what fraction of the cycle is interphase?
M = 24 − (10 + 8 + 4) = 2 h. Interphase = 22 h, so 22/24 ≈ 92% of the time.
Common mistakes
- Thinking chromosomes are copied during mitosis. They are copied earlier, in the S phase of interphase.
- Saying meiosis makes 2 cells. It divides twice and makes 4.
- Thinking a cell with an X-shaped chromosome has two chromosomes. The X is one chromosome made of two sister chromatids.
- Thinking meiosis happens everywhere in the body. It happens only in the organs that make gametes.