Chromosome, a tiny body-contained within the nucleus of a cell of a living organism. When stained with a dye, chromosomes can be viewed with a strong microscope. Chromosomes consist of threadlike structures that are made up of proteins (mostly histones) and nucleic acids (mostly DNA and a small amount of RNA). Each chromosome has subunits called genes, which are the portions of the chromosome that determine hereditary traits. Genes are arranged in a line along the length of the chromosome; there may be thousands of genes on a single chromosome. Because chromosomes play a major role in heredity, they are extensively studied in the branch of biology called genetics.

Each species has a characteristic number of chromosomes in each cell. Chromosomes occur in pairs. A somatic, or body, cell of a human being contains 46 chromosomes, or 23 pairs; of a horse, 30 pairs; of a cotton plant, 26 pairs. A gamete, or sex cell, contains half as many chromosomes as a somatic cell.

Most organisms grow, develop, and reproduce through a process of cell division, which is preceded or accompanied by the division of the nucleus of the cell. During the process of nuclear division, the chromosomes become visibly distinct rod-shaped bodies. At other times, they appear as a tangled mass of threads and granules within the nucleus. Two types of nuclear division occur—mitosis and meiosis.

Somatic cells are produced by mitosis. First, the chromosomes in a cell duplicate themselves. Then, each set of chromosomes moves to opposite sides of the cell and the cell divides. Each new cell has a complete set of chromosomes identical to that in the original cell.

Gametes are produced by meiosis, or reduction division. In meiosis, the number of chromosomes is halved. The male gamete unites with the female gamete during fertilization, and the zygote (fertilized egg) thus has the normal number of chromosomes (and genes) found in somatic cells. In this way the offspring gets half of its chromosomes from the male and half from the female parent.