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. Beadle and Tatum concluded that this mutant was defective somewhere in the biochemical pathway that normally synthesizes arginine. They identified three classes of arginine-deficient mutants, each apparently lacking a key enzyme at a different step in the synthesis of arginine. They demonstrated this by growing these mutant strains in media that provided different intermediate molecules. Their results provided strong evidence for the one gene–one enzyme hypothesis. •Later research refined the one gene–one enzyme hypothesis. •First, not all proteins are enzymes.
Keratin, the structural protein of hair, and insulin, a hormone, both are proteins and gene products. •This tweaked the hypothesis to one gene–one protein.
•Later research demonstrated that many proteins are composed of several polypeptides, each of which has its own gene. •Therefore, Beadle and Tatum’s idea has been restated as the one gene–one polypeptide hypothesis. •Some genes code for RNA molecules that play important roles in cells although they are never translated into protein. Transcription and translation are the two main processes linking gene to protein. •Genes provide the instructions for making specific proteins. •The bridge between DNA and protein synthesis is the nucleic acid...