The Human Body


Chromosome is a microscopic structure within cells that carries the molecule deoxyribonucleic acid (DNA)—the hereditary material that influences the development and characteristics of each organism. In bacteria and bacteria-like organisms called archaebacteria, chromosomes consist of simple circles of DNA floating freely in the organism. In all other life forms, collectively called eukaryotes, chromosomes reside within a well-defined nucleus. In eukaryotes, chromosomes are highly complex structures in which the shape of the DNA molecules is linear, rather than circular.

Chromosomes consist chiefly of proteins and DNA. Tiny chemical subunits called nucleotide bases form the structure of DNA. A sequence of bases along a DNA strand that codes for the production of a protein is known as a gene (see Genetics). Genes occupy precise locations on the chromosome.

Each cell contains enough DNA to form a thread extending about 2 m (about 7 ft). Proteins called histones play a key role in packaging DNA within chromosomes. Sections of the DNA molecule wind around clusters of histones to form units called nucleosomes, which resemble spools encircled with thread. Another type of protein, called nonhistone chromosomal protein, further compresses nucleosomes into a compact, narrow coil. Chromosomes become most condensed when a cell is preparing to divide.

The chromosome structure ensures that even when the DNA is highly confined, it is free to carry out transcription, or the production of messenger ribonucleic acid (mRNA). Messenger ribonucleic acid is the molecule that carries the DNA instructions that determine the types of proteins a cell will reproduce to the sites where proteins are constructed.