Found within the nucleus (the cell's control center), a chromosome consists of tightly coiled DNA wrapped around proteins that give chromosomes their structure. Tinier than a grain of salt, human chromosomes are only between 2 and 10 micrometers in length, but each contains millions of letters of DNA. Each chromosome is like a chapter of a book - brought together, they make up an organism's entire genome and tell a full genetic story.
When a cell divides, it must ensure that both the newly formed cells are given a full set of genetic instructions. As part of this process, the cell's chromosomes, which are otherwise unraveled and diffuse, condense and line up. Researchers can stain these chromosomes and see them through a microscope, stained chromosomes sometimes look like threads with light and dark bands. These bands make it possible to distinguish different chromosomes and line them up to create an organized picture or karyotype.
Human cells contain a total of 46 chromosomes - two sex chromosomes and 22 pairs of non-sex chromosomes. Other species have different numbers of chromosomes and may have more or less than two copies of each. Fruit flies have a mere eight chromosomes, dogs have 78, and goldfish have over 100.