As the Avery-MacLeod-McCarty experiments in the 1940s showed, DNA specifies the cell’s activities. It was now clear that DNA directs the course of cell development and differentiation in multicellular animals, thus forming the individual. It then has a role in maintaining life functions, in repair of damaged cells or tissues, in healing, and in fighting off disease. But scientists did not yet understand how DNA accomplished these activities. Its pivotal role in specifying the sequence of amino acids in proteins was established in the 1960s.
During the decade biochemical methods improved, making it easier to figure out how genes actually worked. The structure of DNA had been established in 1953 by James Watson and Francis Crick, and scientists began unraveling how DNA uses ribonucleic acid, or RNA, to specify the synthesis of proteins. Robert Holley, H. G. Khorana, and Marshall Nirenberg shared the Nobel Prize in 1968 for their key roles in describing this process. They discovered that DNA specifies the structure of RNA and that RNA in turn specifies the structure of protein. DNA works through RNA in specifying protein structure. It does this by way of the genetic code. Three DNA bases in a row specify, through intermediary RNA, a building block of a protein called an amino acid.
The process by which DNA functions to tell the cell how to make its proteins has been called the “central dogma of molecular biology.” Since many proteins are enzymes, whose presence or absence determines whether a certain reaction can occur or not, specification of protein structure is one important way DNA regulates cell activity.
