Deep inside every cell in every living thing are threadlike structures that carry information about that individual. These structures, which are only visible under a microscope, are called chromosomes. The information on the chromosomes is carried in sections or units called genes. When organisms reproduce, their genes are passed along to their offspring. This process is called heredity.

General features

Chromosomes are made up primarily of a material called deoxyribonucleic acid, or DNA. The structure and location of chromosomes within a cell is one of the most important differences between different groups of organisms. In animals and plants, for example, chromosomes consist mainly of DNA, but they also contain ribonucleic acid—RNA—and some proteins. These chromosomes are located inside a structure called the nucleus of the cell. Some organisms, including bacteria, do not have a nucleus. In these organisms, the chromosomes—which are composed only of DNA—float freely within the cell.

Chromosome numbers

Every living thing has a specific number of chromosomes in most of its cells. In most plants and animals, chromosomes come in pairs. These organisms are called diploid organisms. For example, a porcupine's cells each have a total of 34 chromosomes. Because porcupines are diploid organisms, this means they have 17 sets of chromosome pairs, for a total of 34 chromosomes. Potato cells have 24 sets of chromosome pairs, giving them a total of 48. Mosquito cells only have six chromosomes—two sets of three chromosomes. Human cells have 23 sets of chromosomes for a total of 46.

In species that undergo sexual reproduction in order to produce offspring, the number of chromosomes is identical in every type of cell except one—the sex cells. These are the egg cells produced by the ovaries in females and the sperm cells produced by the testes in males. Unlike most of the cells in the body, the sex cells have only one half of each pair of chromosomes. In humans, for example, the sex cells only have 23 chromosomes. In porcupines, the sex cells have only 17 chromosomes.

The reason for this is simple. When it is time for the parents to reproduce, each parent contributes one sex cell. That is, the father contributes a single sperm cell, and the mother a single egg cell. When the sperm and the egg combine, they each contribute their chromosomes to the newly formed embryo. If a porcupine sperm and porcupine egg each had 17 pairs of chromosomes (34 in total) to contribute, then the new embryo would get 34 pairs of chromosomes, or 68 in total—twice as many as it needs. One half of each pair of chromosomes, therefore, comes from the father and one half comes from the mother.