Lecture 33 Genetic code

Lecture 33 Genetic code - Lecture 33 CHAPTER 17 Campbell...

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Lecture 33 CHAPTER 17 Campbell, pages 325 -331 FROM DNA TO PROTEIN: The Genetic Code
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Learning Objectives 1. Describe Beadle and Tatum’s experiments with Neurospora and explain the contribution they made to our  understanding of how genes control metabolism. 2. Distinguish between the “one gene-one enzyme” hypothesis and the “one gene-one polypeptide” hypothesis and  explain why the original hypothesis was changed.  3. Define codon 4. Understand the basis of the early techniques used to crack the genetic code and identify amino acids it specifies. 5. Understand the relationship between the linear sequence of nucleotides and codons on  DNA and on mRNA, and  how these relate to amino acid sequence in a polypeptide.  6. Explain what it means to say that the genetic code is redundant, non-overlapping and unambiguous. 7. Explain the significance of the reading frame during translation. Be able to identify an open reading frame. 8. Be able to identify regions with predicted protein-encoding genes, and provide information about which strand of  DNA is the template, and the approximate length (in amino acids) of the protein.
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WHAT IS A GENE AND HOW DO GENES CONTROL PHENOTYPE? Biochemical genetics: The study of metabolic defects provided evidence that genes specify proteins. In 1901, Sir Archibald Edward Garrod proposed that genes dictate phenotype  through their control of the production of enzymes that catalyze specific chemical  reactions in the cell (~1908). Garrod speculated that inherited alkaptonuria, a disease characterized by the  accumulation of homogenistic acid in urine, was caused by the absence of an enzyme  that breaks down a specific substrate, causing it to accumulate. It took ~5 decades of research to prove Garrod’s hypothesis. PKU Alkaptonuria
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Beadle and Tatum provided experimental proof for the connection between genes and enzymes. Theirs is yet another example of an excellent choice for a model system: Neurospora crassa (bread mold) 1. Cheap 2. Easy to grow in the lab 3. Fungus grows as a haploid for most of its life and therefore makes genetic analyses more straight forward (mutant alleles have a phenotype uncomplicated by dominance/recessivity) Mutations provide a powerful way to determine cause and effect and define biochemical pathways. Beadle and Tatum began what can best be described as “biochemical genetics”.
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EXPERIMENT: 1. Wild type Neurospora  (a “prototroph”) requires only a carbon source, inorganic  salts, and a vitamin.  Neurospora crassa was irradiated with X-rays to induce  mutations in DNA.  2. Arginine  “auxotrophic” mutants were identified by their inability to grow in  minimal medium unless arginine was added.  Beadle and Tatum concluded that  these arg mutants were defective in a gene required for arginine synthesis.
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