Genes were first defined as units of hereditary transmission. The name "gene" was coined by Wilhelm Johannsen in 1909, although the concept of a discrete unit governing inherited characteristics goes back at least to Gregor Mendel in 1861. The work of Thomas Hunt Morgan and his colleagues established that genes were located on chromosomes, and in the mid-1940s Oswald Avery demonstrated that genes were composed of DNA (deoxyribonucleic acid). Since that time, some types of viruses have been discovered that use ribonucleic acid (RNA) instead of DNA, but here we shall concentrate on DNA genes. The discovery of the structure of DNA in 1953 by James Watson and Francis Crick set the stage for the next fifty years of research into gene structure, function, and regulation.
DNA is a linear molecule composed of subunits called nucleotides. Each nucleotide is made of a sugar and phosphate group, plus a chemical base, of which there are four types: adenine, thymine, guanine, and cyto-sine (A, T, G, C). Nucleotides are typically referred to by the name of their base. DNA exists as a pair of strands, wound around one another into a double helix, with the bases directed into the center. The structure and charges of the bases dictate that A on one strand can match only up with T on the other, and C only with G. This complementarity provides the basis for faithful replication of the entire DNA molecule.
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