Gene
Gene, basic unit of heredity found in the cells of all living organisms, from bacteria to humans. Genes determine the physical characteristics that an organism inherits, such as the shape of a tree’s leaf, the markings on a cat’s fur, and the color of a human hair (see Heredity ). Genes are composed of segments of deoxyribonucleic acid (DNA), a molecule that forms the long, threadlike structures called chromosomes. The information encoded within the DNA structure of a gene directs the manufacture of proteins, molecular workhorses that carry out all life-supporting activities within a cell (see Genetics ). Chromosomes within a cell occur in matched pairs. Each chromosome contains many genes, and each gene is located at a particular site on the chromosome, known as the locus. Like chromosomes, genes typically occur in pairs. A gene found on one chromosome in a pair usually has the same locus as another gene in the other chromosome of the pair, and these two genes are called alleles. All