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A chromosome is a package containing a chunk of a genome—that is, it contains some of an organism's genes. The important word here is "package": chromosomes help a cell to keep a large amount of genetic information neat, organized, and compact.

Chromosomes are made of DNA and protein. Most living things have chromosomes that are linear, like bits of fat thread, and are kept in the nucleus, a sphere-shaped sac within the cell.

How many chromosomes do organisms have?

In a few very simple forms of life, such as bacteria, the entire genome is packaged into a single chromosome. But other organisms, with genomes a thousand or even a million times larger than those of bacteria, divide their hereditary material among a number of different chromosomes. Exactly how many chromosomes we are talking about depends on the species. A mosquito has 6 chromosomes, a pea plant has 14, a sunflower 34, a human being 46, and a dog 78.

Closely related species tend to have a similar number of chromosomes. For example, chimpanzees, our closest cousins, have 48 chromosomes in each of their cells. But aside from this general rule, there is very little rhyme or reason to how many chromosomes a species has. It would be convenient if there were some type of "evolutionary ladder," with more complex organisms having more chromosomes, but nature does not work that way. Goldfish, which spend their days swimming in circles, lazily blowing bubbles, and hovering near the top of their bowl in anticipation of fish flakes, are provided with 94 chromosomes. Cats, with their keen hunting techniques and thousand ways to purringly manipulate human beings, have only 38.

What makes one chromosome different from another?

Though similar in basic appearance, different chromosomes vary slightly in size and shape. In addition, when chromosomes are stained with fluorescent dyes they develop distinctive patterns of bright and dark bands. These subtle differences enable cell biologists to distinguish different chromosomes from one another, much as field biologists learn to distinguish members of a pod of whales by the marks and scars on their fins.

The largest chromosome of an organism is generally referred to as chromosome 1, the next largest as chromosome 2, and so on. Different chromosomes contain different genes. That is, each chromosome contains a specific chunk of the genome. For example, in humans the gene for alpha globin, a part of the hemoglobin protein that carries oxygen in red blood cells, is found on chromosome 16. The gene for beta globin, the other part of the hemoglobin protein, is found on chromosome 11.

Stained chromosomes can be photographed and arranged in order of size to produce a karyotype, a chart scientists use to study chromosomes. A karyotype isn't detailed enough to tell you about the individual genes on a chromosome, but it can tell you whether the chromosomes as a whole are in working order and it may help doctors diagnose and understand diseases. For example, people with Down's syndrome have too many chromosomes, and chromosome rearrangements (when a part of a chromosome breaks off and reattaches to a different chromosome) are associated with certain cancers.

A quick glance at any karyotype will tell you one of the most important facts about chromosomes: They come in pairs. The members of a pair are the same size and shape, and they have the same banding patterns. In other words, each person actually possesses two copies of chromosome 1, two copies of chromosome 2, and so on. Human cells contain 23 pairs of chromosomes.

Most of an organism's chromosomes—generally all except for one pair—are called autosomes, which are the same in males and females. Humans have 22 pairs of autosomes.

Many organisms also have a pair of sex chromosomes, which differ between males and females. In humans, a female has two identical sex chromosomes. A male has one sex chromosome that is like those of females, and one that is smaller and differently shaped. In scientific shorthand, the female's sex chromosomes are referred to as XX, and the male's as XY.

The sex chromosomes of many other species have a similar pattern, but it is not the only possibility. In grasshoppers, females have two identical sex chromosomes, while males have only one sex chromosome and are said to be XO. In birds, butterflies, and moths, it is the males that have two identical sex chromosomes—they are XX—and females are XY or XO.

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Updated on January 15, 2003