22Mar_18

22Mar_18 - Biological Science I Tuesdays and Thursdays...

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Biological Science I Tuesdays and Thursdays 8:00-9:15, HCB 102 Mondays 5:15-6:15, KIN 1024 22 March 2011 – Lecture 18 At the end Chapter 17 you should be able to: Describe the contributions made by Garrod, Beadle, and Tatum to our understanding of the relationship between genes and enzymes Briefly explain how information flows from gene to protein Compare transcription and translation in bacteria and eukaryotes Explain what it means to say that the genetic code is redundant and unambiguous Include the following terms in a description of transcription: mRNA, RNA polymerase, the promoter, the terminator, the transcription unit, initiation, elongation, termination, and introns Include the following terms in a description of translation: tRNA, wobble, ribosomes, initiation, elongation, and termination 1. The Flow of Genetic Information The information content of DNA is in the form of specific sequences of nucleotides The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins Proteins are the links between genotype and phenotype Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation How was the fundamental relationship between genes and proteins discovered? ONE GENE ONE PROTEIN a. Archibald Garrod a.i. In 1909, British physician Archibald Garrod first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions a.ii. He thought symptoms of an inherited disease reflect an inability to synthesize a certain enzyme a.iii. Alkaptonuria (black urine disease or alcaptonuria) is a rare inherited genetic disorder of phenylalanine and tyrosine metabolism. This is an autosomal recessive condition that is due to a defect in the enzyme homogentisate 1,2-dioxygenase, which participates in the degradation of tyrosine. As a result, a toxic tyrosine byproduct called homogentisic acid (or alkapton) accumulates in the blood and is excreted in urine in large amounts (hence -uria). Excessive homogentisic acid causes damage to cartilage (ochronosis, leading to osteoarthritis) and heart valves as well as precipitating as kidney stones. a.iv. Linking genes to enzymes required understanding that cells synthesize and degrade molecules in a series of steps, a metabolic pathway b. George Beadle and Edward Tatum b.i. George Beadle and Edward Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal medium as a result of inability to synthesize certain molecules b.ii. Using crosses, they identified three classes of arginine-deficient mutants, each lacking a different enzyme necessary for synthesizing arginine 1
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Biological Science I Tuesdays and Thursdays 8:00-9:15, HCB 102 Mondays 5:15-6:15, KIN 1024 22 March 2011 – Lecture 18 b.iii. They developed a one gene–one enzyme hypothesis, which states that each gene dictates production of a specific enzyme b.iv. Some proteins aren’t enzymes, so researchers later revised the hypothesis: one gene–one
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22Mar_18 - Biological Science I Tuesdays and Thursdays...

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