8 GENE EXPRESSION  DNA, being the genetic material, carries all the informations necessary to programme the functions of a cell by controlling the synthesis of enzymes or proteins.  Beadle and Tatum put forward a theory-one gene one enzyme in support of the earlier hypothesis that enzymes are proteinaceous in nature and each is produced by a single gene.  They conducted experiments on the nutritional strains of pink mould, Neurospora crassa.  This fungus grows on simple nutrient medium and has the ability to synthesize all its cellular components. Such an organism is called prototroph.  An organism that is unable to synthesize a particular cellular metabolite, such as an amino acid or coenzyme is called auxotroph.  Beadle and Tatum produced arginine (an amino acid) auxotrophs (mutants of Neurospora unable to synthesize arginine) by giving X-rays treatment to the cells. Arginine synthesis passes through the following path Gene1  Enzyme 1  Gene 2  Enzyme 2  Gene 3  Enzyme 3  Precursor  Ornithine  Citrulline  Arginine  They found that any step of this metabolic chain could be blocked by a mutation in a specific enzyme catalyzing the reaction, each enzyme representing a different gene product.  Thus, Beadle and Tatum reached a conclusion that each gene functions to produce a single enzyme.  Some proteins, e.g., haemoglobin and other quaternary proteins are made up of two or more than two polypeptide chains. After it, the 'one gene one enzyme' theory was modified into one gene one polypeptide hypothesis by Yanofsky. Later Jacobson and Baltimore proposed one mRNA -one polypeptide hypothesis.  Gene and protein : A gene expresses itself by protein synthesis.  When a particular gene is expressed (i.e., controls a function or a reaction), its information is copied into another nucleic acid, i.e., mRNA (messenger RNA) which in turn directs the synthesis of specific proteins.  These concepts form the central dogma of molecular biology. This has been shown by F. Crick in the following diagram. Fig: Central Dogma  An exception to this one way flow of information was reported in 1970.  H. Temin and D. Baltimore independently discovered reverse transcription in some viruses.  These viruses can code an enzyme reverse transcriptase which can code DNA on RNA  template. This discovery was important in understanding cancer and, hence, these two scientists were awarded Nobel Prize. Reverse flow of transcriptional information or Reverse Central Dogma or Teminism.  Commoner suggests circular flow of information.