You began life as a single cell. Today you have trillions, and every one of them came from that first cell dividing, then dividing again, and again. Cell division is also how a scraped knee heals and how your body replaces the skin cells you rub off every day. The kind of division that makes new body cells is called mitosis, and it produces two daughter cells with exactly the same genetic information as the parent. Getting that right every time takes an orderly sequence of steps called the cell cycle.
Why cells divide instead of just growing bigger
A cell gets everything it needs (oxygen, nutrients) and gets rid of wastes through its cell membrane, its surface. But it uses those materials throughout its whole volume. As a cell grows, its volume grows faster than its surface area. Picture a cube-shaped cell: with sides of 1 unit it has a surface area of 6 square units and a volume of 1 cubic unit, a ratio of 6 to 1. Double the side to 2 units and the surface area becomes 24 but the volume becomes 8, a ratio of only 3 to 1. A large cell would not have enough membrane to supply its interior quickly enough by diffusion, the movement of particles from where they are more concentrated to where they are less concentrated. Dividing into two smaller cells restores a high surface-area-to-volume ratio. One nucleus can also only direct a limited amount of cytoplasm.
- Growth: an organism gets bigger by making more cells, not mainly by making cells bigger.
- Repair: new cells replace damaged ones, for example when a cut heals or a broken bone mends.
- Replacement: cells that wear out are replaced; the lining of your intestine is renewed every few days.
- Asexual reproduction: some organisms, such as many single-celled organisms and plants grown from cuttings, reproduce by mitosis.
The stages of the cell cycle
A chromosome is a long strand of DNA wrapped around proteins. Most human body cells have 46. During interphase the cell grows, carries out its normal job and copies every chromosome. Each copied chromosome is made of two identical sister chromatids joined at a point called the centromere. Interphase is usually the longest part of the cycle; in many actively dividing cells it takes about 90% of the time. Mitosis then divides the copied chromosomes evenly, and cytokinesis divides the cytoplasm.
| Stage | What happens | How to recognize it |
|---|---|---|
| Interphase | Cell grows, does its normal work and copies its DNA | A clear nucleus with a nuclear membrane; chromosomes not visible as separate strands |
| Prophase | Chromosomes coil up and become visible; the nuclear membrane breaks down; spindle fibres start to form | Thick, thread-like chromosomes scattered in the cell; no clear nuclear membrane |
| Metaphase | Spindle fibres attach to each centromere and line the chromosomes up across the middle (equator) of the cell | Chromosomes in a single line across the centre |
| Anaphase | Sister chromatids separate and are pulled to opposite ends (poles) of the cell | Two groups of V-shaped chromosomes moving apart |
| Telophase | Chromosomes reach the poles and uncoil; a nuclear membrane forms around each set; the spindle disappears | Two nuclei forming at opposite ends of one cell |
| Cytokinesis | The cytoplasm divides, making two separate daughter cells | Animal: cell pinches in (cleavage furrow). Plant: a cell plate forms across the middle |
- Centrioles at each pole help organize the spindle
- Cytokinesis: the membrane pinches inward to form a cleavage furrow
- Dividing cells found in many tissues, such as skin, bone marrow and the intestinal lining
- Usually no centrioles (flowering plants have none); the spindle still forms
- Cytokinesis: a cell plate grows across the middle and becomes a new cell wall, because the rigid wall cannot pinch in
- Dividing cells found mainly in meristems, such as root tips and shoot tips
Estimating how long each stage lasts
In a prepared slide of an onion root tip, the cells were all frozen at random moments in their cycles. A stage that lasts a long time catches many cells; a short stage catches only a few. So the fraction of cells in a stage is a good estimate of the fraction of the cycle that stage takes. This is a classic investigation for this unit, and laboratory technologists use the same idea when they look for unusually high numbers of dividing cells in a tissue sample.