Like a combination of paper clip, duct tape, and sheepdog, proteins keep the DNA folded into an orderly, compact shape. A chromosome may look simple at first glance, but it is actually quite a complex structure, with the DNA molecule wound around protein spools and fastened into loops, coils, and fibers by other proteins. In a chromosome, protein is the packaging and DNA is the contents of the package. Generally, however, chromosomes are fully condensed only in preparation for cell division. The rest of the time, some of the loops and coils are unfastened so that the DNA can do its work, communicating hereditary instructions to the rest of the cell.
New cells get their chromosomes from old cells through cell division, or mitosis.
In fact, mitosis is the chromosomes' real opportunity to shine. Viewed under a microscope, the chromosomes in a dividing cell seem to perform a series of stylized, dancelike movements, which have fascinated researchers for more than a century. In preparation for mitosis, a cell makes a copy of each of its chromosomes, building replicas of each DNA molecule and the packaging proteins. Once they have all taken their places, the chromosome pairs quite rapidly move apart, as if some powerful chromosome magnet had been placed on each side of the cell. Limbs trailing, the chromosomes move towards opposite ends of the cell, forming two clumps that are separated when the cell divides.