Chromosome << KROH muh sohm >> is a tiny, thin, threadlike structure found in cells of all organisms. Chromosomes are the carriers of inheritance—that is, the physical or behavioral characteristics offspring receive from parents. The passing of such characteristics is called heredity. The basic unit of inheritance is the gene. Genes are long sequences of DNA (deoxyribonucleic acid) molecules that make up most of the material in chromosomes. Proteins called histones bend and coil the long DNA molecules to form chromosomes.
Chromosomes come in various lengths and shapes, depending on the organism. Their characteristic shape allows scientists to identify individual chromosomes when viewed under a microscope. Every species (type) of organism has a certain number of chromosomes in each cell of its body. Human body cells, except for sperm or egg cells, typically have 46 chromosomes.
Chromosomes have an essential role in cell division. New cells are formed by division, so that there are two cells, often called daughter cells, where there once was only one cell. Before division begins, the chromosomes are duplicated. During division, each duplicated chromosome forms into a pair of rodlike structures. The new cells that are formed receive one of each pair of chromosomes. The new cells then each have a set of chromosomes exactly like those of the original cell.
For much of the time, a cell’s chromosomes are partially uncoiled so that the genes can produce the various biological molecules necessary for cell function. When cells divide, the chromosomes condense into a compact state. As a result, different sections of DNA do not become entangled as the chromosomes are split between the two daughter cells. On rare occasions, a fragment of a chromosome can break off and join to another chromosome. This process, called rearrangement, can cause problems in the development or proper functioning of cells. In human beings, rearranged chromosomes can cause certain diseases that can be passed on from a parent to a child. Many cancer cells also have changes in their chromosome number or structure.
Walter Flemming, a German scientist, first observed the role of chromosomes in cell division in 1882. He noticed that when salamander cells divided, the chromosomes were duplicated and divided equally into the two daughter cells.
