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Chapter 15

Chapter 15 Textbook Notes - How Do Genes Work

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University of Toronto Mississauga
Fiona Rawle

Notes From Reading CHAPTER 15:H OW G ENES W ORK (PGS.330-341) Chapter 15 – How Genes Work Key Concepts - Most genes code for proteins. - In cells, information flows from DNA to RNA to proteins. DNA is transcribed by RNA polymerase to messenger RNA, and then messenger RNA is translated by ribosomes to proteins. - Each amino acid in a protein is specified by a group of three bases in RNA. What Do Genes Do? - Early advances showed that genes carry the instructions for making and maintaining an individual - In order to infer what a particular gene does, George Beadle and Edward Tatum damaged the gene in the bread mold Neurospora crassa, creating a knock-out mutant - They then observed the effect this had on the phenotype of the mutant - The idea was to knock out a gene by damaging it and then infer what the genes does by observing the phenotype of the mutant individual - Out of this work came their one-gene, one-enzyme hypothesis, which proposed that genes contain the information needed to make proteins, many of which function as enzymes - Based on analyses of knock-out mutants, the one-gene, one-enzyme hypothesis claimed that genes contain the information needed to make proteins, many of which function as enzymes - Metabolic pathway – organisms synthesize in a series of steps - To find the mutant s they were looking for, the researchers performed what is now known as a genetic screen - A genetic screen is a technique for picking certain types of mutants out of many thousands of randomly generated mutants - Later scientists performed genetic screens for N. crassa mutants defective in the synthesis of the amino acid arginine and then analysized the step at which this synthesis was blocked - Their results provided support for the one-gene, one-enzyme hypothesis 15.2 The Central Dogma of Molecular Biology - Francis Crick proposed that the sequence of bases in DNA is a kind of code and that DNA is an information storage molecule - Different combinations of bases could specify the 20 amino acids - A particular stretch of DNA, then could contain the information needed to specify the amino acid sequence of a particular enzyme - The information encoded in the base sequence of DNA is not translated into the amino acid sequence of proteins directly - Instead, the link between DNA as information repository and proteins as cellular machines is indirect Notes From Reading CHAPTER 15:H OW G ENES W ORK (PGS .330-341) RNA as the Intermediary between Genes and Proteins - The first clue that biological information does not flow directly from DNA to proteins came from the data on the structure of cells - Observation began to make sense after Francois Jacob and Jacques Monod suggested that RNA molecules act as a link between genes and the protein-manufacturing centres - They predicted that short-lived molecules of RNA called messenger RNA, or mRNA carry information from DNA to the site of protein synthesis - RNA polymerase polymerizes ribonucleotides into strands of RNA - The key observation was that RNA polymerase synthesizes RNA molecules according to the information provided by the sequence of bases in a particular stretch of DNA - Unlike DNA polymerase, RNA polymerase does not require a primer to begin adding ribonucleotides to a growing strand of RNA The Central Dogma - The central dogma summarizes the flow of information in cells - It simply states that DNA codes for RNA, which codes for proteins: o DNA  RNA  proteins - The sequence of bases in a parti
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