Image courtesy of the US National Library of Medicine
Chromosomes are the packages that hold DNA. They play an essential role in genetics - they are the parcels of genetic material that are passed down from parents to children and together contain the full set of inherited instructions that an embryo receives.
Found within the nucleus (the cell's control center), a chromosome consists of tightly coiled DNA wrapped around proteins that give chromosomes their structure. A single chromosome has two segments - called the p arm (longer) and the q arm (shorter) - which are pinched together. In many organisms, chromosomes are found in pairs, and they are often depicted in an "X" shape, drawn together at the intersection of the p and q arms. 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.
Image courtesy of Dr. Cynthia Morton, Center for Advanced Molecular Diagnostics, Cytogenetics Laboratory
When cells copy and divvy up their chromosomes during cell division, errors can happen - sometimes the wrong amount of genetic material gets passed along. If an error occurs in the egg or sperm, every cell in the body of the offspring will contain this abnormality. Occasionally, an entire chromosome is missing or an extra one is present. (Down's syndrome occurs when a person has an extra copy of chromosome 21.) Sometimes, a portion of a chromosome is missing (known as a "deletion") or present in excess copies ("duplication"); a segment of a chromosome may break off and attach to a different chromosome ("translocation") or break off, flip around, and reattach ("inversion"); or a portion may break off and form a small circle ("ring"). Cancer cells contain lots of these chromosomal mix-ups as mutations accumulate in these degenerate cells.
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.
Learn how karyotypes can be used to predict genetic disorders and test your karyotype skills by matching partner chromosomes at University of Utah's Genetic Science Learning Center. You can learn more about the central role that centromeres (the structures that join the two arms of a chromosome) play in cell division from the Wellcome Trust.
Curious about chromosome numbers? Find out the number of chromosomes for different species at this Rutgers University site.
You can also explore the human chromosomes and find out what genes reside on each one using the chromosome browser at the Wellcome Trust's Interactive Centre.