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For more than 50 years after the science of genetics was established and the patterns of inheritance through genes were clarified the largest questions remained unanswered How are the chromosomes and their genes copied so exactly from cell to cell and how do they direct the structure and behavior of living things This paper will discuss those questions and the people that answered them Two American geneticists George Wells Beadle and Edward Lawrie Tatum provided one of the first important clues in the early 1940s Working with the fungi Neurospora and Penicillium they found that genes direct the formation of enzymes through the units of which they are composed Annas 1996 Each unit a polypeptide is produced by a specific gene This work launched studies into the chemical nature of the gene and helped to establish the field of molecular genetics The fact that chromosomes were almost entirely composed of two kinds of chemical substances protein and nucleic acids had long been known Partly because of the close relationship established between genes and enzymes which are proteins protein at first seemed the fundamental substance that determined heredity Goetinck 1995 In 1944 however the Canadian bacteriologist Oswald Theodore Avery proved that deoxyribonucleic acid DNA performed this role He extracted DNA from one strain of bacteria and introduced it into another strain The second strain not only acquired characteristics of the first but passed them on to subsequent generations By this time DNA was known to be made up of substances called nucleotides Each nucleotide consists of a phosphate a sugar known as deoxyribose and any one of four nitrogen-containing bases The four nitrogen bases are adenine A thymine T guanine G and cytosine CCaldwell 1996 In 1953 putting together the accumulated chemical knowledge geneticists James Dewey Watson of the US and Francis Harry Compton Crick of Great Britain worked out the structure of DNA This knowledge immediately provided the means of understanding how hereditary information is
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