Why do cells divide?
Quick answer
Cells divide so living things can grow, repair injuries, replace old or worn-out cells, and reproduce. Most body cells divide by mitosis, making two identical copies. Sex cells (eggs and sperm) are made by meiosis, which halves the number of chromosomes.
You started as a single cell. Today your body has an estimated 30 trillion or more human cells, and every one of them came from cell division.
Four reasons cells divide
| Reason | Example |
|---|---|
| Growth | A baby grows into an adult by adding cells, not by making cells bigger |
| Repair | Skin cells divide to close a cut; bone cells help heal a fracture |
| Replacement | The lining of your gut is replaced every few days; red blood cells last about 120 days |
| Reproduction | Bacteria split in two to make new organisms; humans make eggs and sperm |
Why not just grow bigger cells?
A cell takes in food and oxygen and gets rid of waste through its outer membrane. As a cell grows, its volume increases faster than its surface area, so a very large cell couldn’t move materials in and out fast enough. Dividing keeps cells small and efficient. The cell’s DNA also can only control so much material.
Mitosis vs meiosis
| Mitosis | Meiosis | |
|---|---|---|
| Purpose | Growth, repair, replacement | Making eggs and sperm |
| Number of new cells | 2 | 4 |
| Chromosomes in each new cell (humans) | 46 (same as parent) | 23 (half) |
| Genetically identical? | Yes | No, each is unique |
Before a cell divides, it copies its DNA so each new cell gets a full set of instructions. Those instructions are used to build proteins from amino acids.
When division goes wrong
Cells normally divide only when they get the right signals and stop when they should. When those controls break down, cells can divide uncontrollably, which is how cancer begins. That’s why scientists study the cell cycle so closely.
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