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Chapter 17 Notes


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P a g e | P a g e | [ CAMPBELL BIOLOGY: CHAPTER 17 NOTES ] Chapter 17: From Gene to Protein Overview: The Flow of Genetic Information The information content of DNA is in the form of specific sequences of nucleotides along the DNA strands. The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins. Gene expression, the process by which DNA directs protein synthesis, includes two stages called transcription and translation. Proteins are the links between genotype and phenotype. ° For example, Mendel’s dwarf pea plants lack a functioning copy of the gene that specifies the synthesis of a key protein, gibberellin. ° Gibberellins stimulate the normal elongation of stems. Concept 17.1 Genes specify proteins via transcription and translation The study of metabolic defects provided evidence that genes specify proteins. In 1909, Archibald Gerrod was the first to suggest that genes dictate phenotype through enzymes that catalyze specific chemical reactions in the cell. ° He suggested that the symptoms of an inherited disease reflect a person’s inability to synthesize a particular enzyme. ° He referred to such diseases as “inborn errors of metabolism.” Gerrod speculated that alkaptonuria, a hereditary disease, was caused by the absence of an enzyme that breaks down a specific substrate, alkapton. ° Research conducted several decades later supported Gerrod’s hypothesis. Progress in linking genes and enzymes rested on the growing understanding that cells synthesize and degrade most organic molecules in a series of steps, a metabolic pathway. In the 1930s, George Beadle and Boris Ephrussi speculated that each mutation affecting eye color in Drosophila blocks pigment synthesis at a specific step by preventing production of the enzyme that catalyzes that step. ° However, neither the chemical reactions nor the enzymes that catalyze them were known at the time. Beadle and Edward Tatum were finally able to establish the link between genes and enzymes in their exploration of the metabolism of a bread mold, Neurospora crassa . ° They bombarded Neurospora with X-rays and screened the survivors for mutants that differed in their nutritional needs. ° Wild-type Neurospora can grow on a minimal medium of agar, inorganic salts, glucose, and the vitamin biotin. Beadle and Tatum identified mutants that could not survive on minimal medium, because they were unable to synthesize certain essential molecules from the minimal ingredients. ° However, most of these nutritional mutants can survive on a complete growth medium that includes all 20 amino acids and a few other nutrients. One type of mutant required only the addition of arginine to the minimal growth medium.
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