their cellular environment. Chaperone proteins work to stabilize newly synthesized proteins. They ensure the new protein folds into its correct functional conformation in addition to making sure products do not aggregate in areas where they should not. Proteins can also function as enzymes, increasing the rate of various biochemical reactions and turning substrates into products. Products can be modified by attaching groups such as phosphate via an enzyme to specific amino acids in the primary sequence. Proteins can also be used to move molecules in the cell to where they are needed, these are called motor proteins. The shape of the cell is supported by proteins. Proteins such as actin, microtubules and intermediate filaments provide structure to the cell. Another class of proteins are found in plasma membranes. Membrane proteins can be associated with the plasma membrane in different ways, depending on their structure. These proteins allow the cell to import or export cell products, nutrients or signals to and from the extracellular space. Other proteins help the cell to perform regulatory functions. For example, transcription factors bind to DNA to help transcription of RNA. Historical background In 1941, Beadle and Tatum proposed on the basis of their study of mutants of the fungus Neurospora sitophila that genes control specific biochemical reactions. They suggested that the functioning of an organism depends on an integrated system of chemical reactions controlled in some manner by genes. They further noted that “It is entirely tenable to suppose that these genes, which are themselves a part of the system, control or regulate specific reactions in the system either by acting directly as enzymes or by determining the specificity of enzymes.” This line of reasoning gave rise to the “one gene–one enzyme hypothesis”. In a retrospective article, Beadle discussed the status of the one gene-one enzyme hypothesis 10 years after it was proposed. Beadle commented on the Cold Spring Harbor Symposium meeting of biologists in 1951. He noted “I have the impression that the number whose faith in one gene-one enzyme remained steadfast could be counted on the fingers of one hand—with a couple of fingers left over.” However by the early 1960s the concept that the DNA base sequence of a gene specifies the amino acid sequence of a protein became well established on the basis of numerous experiments. For example, an experiment by Crick, Brenner, Barnett and Watts-Tobin in 1961 demonstrated that each