Chromosomes are composed of segments of DNA wound tightly around proteins called histones. Compaction of DNA around the histones during cell division makes chromosomes visible under a microscope.
DNA contains the instructions for making proteins necessary for cellular function. It is loosely coiled in the nucleus of each cell. Chromosomes package this coiled DNA into a structure that can be easily moved during cell division. Cell division is necessary for growth and to replace old or damaged cells.
The appearance of chromosomes varies among species. Some species have chromosomes that are linear and arranged in pairs, while other species have chromosomes with a circular shape.