Cell division: mitosis and growth
Biology · Key Stage 3 · The School
Why organisms need cell division
An organism starts life as a single fertilised cell. Every cell in a fully grown body descends from that one cell through repeated CELL DIVISION. Growth is mostly an increase in cell NUMBER, not cell size. Early growth comes mainly from making more cells, although later some cells also get bigger.
Mitosis produces identical copies
MITOSIS is the type of cell division used for growth and repair. One cell divides into two new cells, each with a complete, identical copy of the original cell's DNA. This is why replacing skin cells or healing a cut does not change what type of tissue is being replaced — the new cells are exact copies.
Cell division is not only for growing bigger
Besides growth, mitosis is used for REPAIR, replacing damaged or worn-out cells such as skin or blood cells throughout life, and for ASEXUAL REPRODUCTION in some organisms, where a whole new organism can form from cells produced this way. The same process — one cell becoming two identical cells — serves several different purposes depending on where and when it happens.
Imagine one fertilised cell that divides by mitosis, and then every new cell divides again, round after round. Each round doubles the number of cells. After round 1 there are 2 cells, then 4, 8 and 16, and 32 after round 5. After round 10 there are 1,024, because ten 2s multiplied together make 1,024. How many rounds could make a whole body? A 2016 paper by Sender, Fuchs and Milo estimates about 30 trillion (30 million million) human cells in a 70 kg adult man. Other sources give different estimates, so this is a rough guide. Keep doubling on a calculator. After round 44 there are about 17.6 trillion cells, which is too few. After round 45 there are about 35.2 trillion, which is enough. Even when every cell divides in every round, 44 rounds are not enough, so at least 45 rounds of mitosis are needed. Repeated doubling reaches huge numbers of cells in few rounds.