In 1909, Wilhelm Johannsen (1857–1927), a Danish biologist, first proposed the name gene as the term designating the basic unit of information that is inherited. In 1944, the Canadian bacteriologist Oswald T. Avery (1887–1955) and American scientists Colin M. Macleod (1909–1972) and Maclyn McCarty demonstrated that DNA is the material responsible for a process called transformation in bacteria, or the transfer of genetic information from one bacterium to another. These researchers
Strands of DNA. Photograph by Howard Sochurek. Stock Market. Reproduced by permission.
had no idea how important their discoveries were until many years later when further studies demonstrated that DNA was the material responsible for the transfer of genetic information in most living organisms. James Watson, an American biochemist, and Francis Crick, a British scientist, presented in 1953 a model of DNA that resembles a twisted ladder. The sides of the ladder are composed of sugar-phosphate groups, and the rungs consist of paired nitrogenous bases. It was shown that there are bases in DNA and the arrangement of the four bases encodes the information held by genes. The DNA model explains how DNA replicates, or makes copies of itself. Later, the American biochemist Marshall W. Nirenberg, and others, used the model to work out the genetic code—the relationship between the arrangement of the DNA bases and the amino acids produced by the DNA sequences in each gene.