Cell division and cancer

Why cells divide

Remember the cell theory: new cells only come from existing cells. Cell division is how that happens, and multicellular organisms use it for three things:

• Growth. A baby becomes an adult by making more cells, not bigger ones.
• Repair. Cuts heal, broken bones knit and lost skin is replaced by new cells.
• Replacement. Worn-out cells are replaced. A red blood cell lasts about four months; cells lining your stomach last only a few days.

Unicellular organisms also divide to reproduce. A bacterium simply copies its loop of DNA and splits in two, called binary fission. In ideal conditions some bacteria do this every 20 minutes, so one bacterium could become more than 2 million in 7 hours. That's why food left out on a warm day can make you sick.

Mitosis: making identical copies

Before a cell divides, it has to make sure each new cell gets a full set of instructions. The DNA in the nucleus is packaged into thread-like chromosomes; human body cells have 46. The division of the nucleus that shares them out equally is called mitosis.

1. The DNA copies itself. Each chromosome is copied, so the two copies sit joined together.
2. Chromosomes line up across the middle of the cell. Fibres attach to each one.
3. The copies are pulled apart, one copy of each chromosome to each end of the cell.
4. The cell splits in two. A new nucleus forms around each set and the cytoplasm divides. In plant cells a new cell wall forms across the middle.

The result is two daughter cells, identical to each other and to the parent cell, with exactly the same DNA. A skin cell makes skin cells; a liver cell makes liver cells.

Keeping division under control

Your body controls exactly when cells divide. Cells check their DNA for damage before dividing, and they stop dividing when they are packed in tightly against their neighbours. When you cut yourself, cells at the edge of the wound sense the space, divide to fill it, and stop once the gap is closed.

DNA can be damaged, for example by UV light from the Sun, by chemicals in cigarette smoke, or just by copying mistakes. A cell with badly damaged DNA normally destroys itself in a tidy, controlled way called apoptosis ("programmed cell death"). Billions of your cells do this every day, and it is how webbing between your fingers disappeared before you were born.

Cancer: division out of control

Cancer starts when a cell's DNA is damaged in the genes that control division, and the cell escapes apoptosis. It keeps dividing even when it is crowded, and all its daughter cells inherit the same faults. The growing lump of cells is a tumour.

• A benign tumour stays in one place and doesn't invade other tissues. It can usually be removed by surgery, though it can still cause harm if it presses on something important, such as in the brain.
• A malignant tumour invades nearby tissues, and cells can break off and travel in the blood to start new tumours elsewhere in the body. This spread is what makes cancer so dangerous.

Treatments include surgery to cut tumours out, radiotherapy (radiation aimed at the tumour), chemotherapy (drugs that kill dividing cells, which is why they can also make hair fall out), and newer medicines that help the immune system recognise cancer cells.

You can lower your risk of some cancers: be sun-smart, don't smoke, eat plenty of fruit and vegetables, stay active, and get vaccinated.