Chromosome
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 pack