Every cell in your body traces back to a single fertilised egg, dividing over and over — and almost all of those divisions use one process, mitosis, built to copy a cell's full set of chromosomes exactly, producing two genetically identical daughter cells. A small, specialised set of cells uses a completely different process instead, meiosis, built to do the opposite: deliberately shuffle and halve the genetic material it copies.
Mitosis: one copy in, two identical copies out
Mitosis takes a single cell with a full set of chromosomes, duplicates that full set once, then divides the cell once, producing two daughter cells, each with a complete, identical copy of the original chromosome set. This is how a body grows, replaces worn-out skin and blood cells, and heals a wound — genetically identical replacement, over and over, for as long as an organism lives.
Meiosis: the process built to make every result different
Meiosis, by contrast, is used only to produce sperm and egg cells, and it works specifically to introduce variation rather than avoid it. It involves two rounds of division rather than one, halving the chromosome number so that a sperm and egg can combine to restore the full count at fertilisation — but along the way, a step called crossing over lets matching chromosomes physically exchange segments of genetic material with each other, and a separate step called independent assortment randomly shuffles which chromosome from each matching pair ends up in which resulting cell. The result is that no two sperm or egg cells from the same person carry an identical combination of genetic material — meiosis is a machine specifically built to produce genetic difference, not to prevent it.
What we're still unsure about
Crossing over and independent assortment together produce an enormous number of possible genetic combinations, but exactly how a cell decides where along a chromosome crossing over will happen — and what regulates how many crossover events occur per chromosome pair — is still an active area of research. Errors in this process, chromosomes failing to separate correctly, called nondisjunction, are a major, still only partly understood cause of genetic conditions and miscarriage, making the precise mechanics of meiotic fidelity a genuinely open topic in current cell biology, not a fully solved textbook process.
This sits inside Cell Division: Mitosis & Meiosis, one of seven topics in Cell Biology, one of six domains in Biology, one of seventeen subjects the app can quiz you on.