Deoxyribonucleic Acid, or DNA, is the basic unit of genetic instruction. DNA is a double helix made up of two strands of phosphates and ribose sugars in alternation, connected by the bases adenine, thymine, cytosine, and guanine.
DNA is a long and fragile polymer in its raw state. Packing it into a more compact structure allows it to fit into the nucleus, protects it from damage, and allows cellular control. Strands of DNA wrap around histone proteins, forming structures known as nucleosomes. This creates chromatin, a level of structure that is often compared to beads on a string. Chromatin then folds again with the assistance of the H1 histone, becoming more compact. Scaffolding proteins cause more folding and twisting, completing the structure we now call a chromosome.
Chromosomes that are preparing to undergo cell division incorporate additional scaffolding proteins which create more folding, which makes the chromosomes more compact and less liable to break.