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Role of DNA Proteins • Order of bases in one gene codes for a protein • Proteins are often used in the cell as enzymes • Different enzymes cause different cells to behave differently What happens if the enzyme is wrong? • Phenylketonuria (PKU) – Due to deficiency in one enzyme (phenylalanine hydroxylase) – Correct enzyme converts phenylalanine tyrosine – Discovered in 1934 by Asbjørn Følling – Gene located on chromosome 12 – If gene not present, enzyme lacking, and phenylalanine toxic phenypyruvate What happens? • Phenylpyruvate builds up in the body • Causes mental retardation, heart disease, death • Occurrence is about 1 in 15 000 births • About 2% of US population carries the gene (heterozygous) • Heterozygous individual can make normal enzymes Beadle and Tatum • George Beadle and Edward Tatum, 1941 • Exposed spores of Neurospora crassa (a bread mold) to X-rays or UV radiation and studied the resulting mutations • Each mutant has specific nutrition needs • Discovered that each mutant strain differed by only one gene • Beadle and Tatum were able to create mutants that could not form specific enzymes • Formulated the “one gene one enzyme” hypothesis – Each gene is responsible for building one enzyme One gene, one protein • Linus Pauling separated different forms of hemoglobin (normal and sickled cells) • Discovered that the two forms differ by only one amino acid (out of 300) • Further investigation determined that a gene is responsible for coding for the order of amino acids that make up one polypeptide • Proteins are polymers of polypeptides Genes code for • Proteins! • Read pages 237-242