Why Do Cells Divide?
Every multicellular organism starts as a single cell (a fertilised egg) and grows to trillions of cells. Cell division is also needed to:
- Replace worn-out cells (the lining of the gut replaces itself every 3–5 days; red blood cells last about 120 days)
- Repair damaged tissue (skin heals after a cut)
- Produce sex cells (eggs and sperm) for reproduction
The Cell Cycle
Before a cell can divide, it must copy all its DNA so that both new cells receive a complete set of genetic instructions. The key stages are:
- Interphase — the cell grows and duplicates its DNA (this is not division itself, but preparation)
- Division — the cell splits into two
Mitosis — Producing Identical Cells
Mitosis is the type of cell division used for growth and repair. It produces two daughter cells, each genetically identical to the parent cell (same number of chromosomes, same DNA).
Key stages:
- DNA condenses into visible chromosomes (each chromosome is a double copy)
- Chromosomes align in the centre of the cell
- Copies separate — one set is pulled to each end of the cell
- Two new nuclei form; the cell divides in two
Result: 2 identical cells, each with the same chromosome number as the original.
Your body performs about 3.8 million cell divisions every second — mostly to replace red blood cells, gut lining cells, and skin cells that constantly wear out. That is roughly 330 billion new cells per day.
Meiosis — Producing Sex Cells
Meiosis is used only to produce egg and sperm cells (gametes). It produces four daughter cells, each with half the original number of chromosomes. This is necessary so that when egg and sperm fuse during fertilisation, the resulting cell has the right total number of chromosomes.
Key difference from mitosis: the chromosome number is halved.
In humans: body cells have 46 chromosomes (23 pairs); egg and sperm cells have only 23 chromosomes.
Uncontrolled Cell Division — Cancer
When the controls on cell division break down, cells can divide uncontrollably, forming a tumour. This is cancer. Cancer cells do not respond to the normal signals that tell cells when to stop dividing.
Summary: Mitosis = growth and repair (2 identical daughter cells, full chromosome number). Meiosis = sex cells (4 daughter cells, half chromosome number). Both start with DNA duplication.
Real world: Skin healing after a cut
When you cut your finger, the skin cells around the wound begin dividing rapidly by mitosis to replace the damaged tissue. Within days, new identical skin cells fill the wound. This is mitosis in action — the new cells must be genetically identical to the originals to produce normal, healthy skin rather than scar tissue.
Real world: Cancer treatment
Cancer occurs when cell division goes wrong and cells divide uncontrollably. Chemotherapy drugs work by targeting cells that are dividing rapidly — both cancer cells and some healthy rapidly-dividing cells, which is why hair loss and nausea are common side effects. Radiation therapy uses radiation to damage the DNA of cancer cells, stopping mitosis. Understanding cell division is central to treating cancer.
Real world: IVF and embryo development
In in vitro fertilisation (IVF), a single fertilised egg cell (zygote) is cultured in a laboratory and begins dividing by mitosis. After just a few days, the cell has divided into a small ball of dozens of identical cells. Embryologists monitor this cell division to select the healthiest embryo for implantation. All of the trillions of cells in the resulting human body trace back to those early mitotic divisions.
Where do you see this in real life? Your body replaces around 330 billion cells every day through mitosis — mostly red blood cells, gut lining, and skin. Every time you see a scab form or notice a cut healing, you are watching mitosis at work. Cancer headlines in the news are also all about cell division — what happens when it goes wrong.