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 — a fully grown adult has roughly the same size cells as a child, just vastly more of them.
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.
Over a single day, an adult human uses mitosis for several different purposes at once, all through the identical process of one cell copying its DNA and dividing into two. Skin cells at the surface are constantly worn away and replaced. Red blood cells, which only survive for a few months, are continuously replaced from bone marrow. A small cut is repaired by cells dividing to fill the gap. None of this is "growth" in the sense of getting taller or heavier — the person is not growing at all — yet all of it depends on exactly the same cell-division process that made them grow in the first place.