Mitosis: making genetically matching cells — Biology, 14–17 years
Mitosis is the cell-division process used for growth, repair and replacement. One parent cell copies its DNA and divides so that two daughter cells receive matching genetic information.
What mitosis does
Before dividing, a cell copies every DNA molecule so there are two matching sets. It then separates the copied chromosomes and splits its cytoplasm, forming two cells. Each daughter cell normally has the same chromosome number and genetic instructions as the original.
Why mitosis is needed
A growing body needs more cells, and damaged tissues need replacements. Mitosis solves both problems while keeping the cell instructions consistent, so a skin cell can be replaced by another skin cell rather than receiving a random genetic set. It also allows many single-celled organisms to reproduce.
Worked example: a healing cut
Suppose a skin cell has 46 chromosomes. First, it copies its DNA, creating two versions of each chromosome. Next, the copies move to opposite sides of the cell. Finally, the cell divides, producing two new skin cells, each with 46 chromosomes and matching instructions.
Trap: copying DNA doubles the chromosome number
This mistake makes sense because a copied chromosome briefly looks like two joined chromosomes, often drawn as an X. But chromosome number counts the joined units, not the individual DNA strands. After copying, the cell has twice as much DNA, yet the same chromosome number until the copies separate.
Where mitosis appears
Mitosis is active when a broken skin surface closes, a child grows taller or the lining of the intestine renews itself. Cancer can arise when cells divide repeatedly without the normal controls. Understanding mitosis helps explain both ordinary healing and why some tumours grow.
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