chromosome

A chromosome is a single, very long strand of DNA that a cell winds up tightly and packs into one neat bundle. It is how a cell keeps its genes folded up, labeled, and out of a tangle, so the instructions for building and running you stay organized in a space far too small to hold a loose thread that long.

To fit, the DNA coils around little spool-shaped proteins, the way thread wraps around a bobbin. Most human cells carry 46 chromosomes — 23 pairs, with one of each pair coming from each parent. The genes living along these strands are your inherited instruction book.

The packing has a practical payoff: when a cell divides, it can copy each chromosome and hand exactly one full set to each new cell. Bundled this way, the genes get shared out evenly and cleanly, instead of spilling everywhere like loose string.