Cell Division

You began as one cell. That cell held a full instruction set, and it did something remarkable: it copied every instruction, split in half, and handed a complete set to each of the two cells it became. Those two did it again, and again, and the repetition never really stopped. The skin you shed this morning is being replaced right now by the same process, running quietly in a layer of cells you will never see.

This lesson follows that process in two very different versions. One makes copies so faithful that the new cells are indistinguishable from the old, and it builds bodies and keeps them stocked. The other deliberately halves the instruction set, and without it sexual reproduction would double the chromosome number in every generation until nothing worked. You will learn what a chromosome actually carries, why body cells hold them in pairs, where each kind of division happens, what comes out of it, how fertilisation puts the pairs back together, and why a single fertilised egg can end up producing cells as unalike as a nerve cell and a red blood cell.