CHAPTER 14 - CHAPTER 14 GENE REGULATION GENE REGULATION IN...

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CHAPTER 14 GENE REGULATION GENE REGULATION IN BACTERIA AND EUKARYOTES: AN OVERVIEW The primary requirement of bacterial gene regulation is the production of enzymes and other proteins o Transcriptional-level control is the most efficient mechanism Although transcriptional-level control is predominant in eukaryotes, control at other levels of gene expression is also very important o Eukaryotic gene regulation is complex o Instead of making new enzymes when they are necessary, preformed enzymes and other proteins are rapidly converted from an inactive to an active state The selective advantages of cell cooperation in multicellular eukaryotes far outweigh the detrimental effects of carrying a load of inactive genes through many cell divisions o Some cells have genes that other cells are missing GENE REGULATION IN BACTERIA Constitutive genes are constantly transcribed o Said to be constitutively expressed Cell metabolic activity is controlled in two ways o Regulating the activity of certain enzymes o Regulating the number of enzyme molecules present in a cell Operons in bacteria facilitate the coordinated control of functionally related genes François Jacob and Jacques Monod noticed that a single genetic defect wiped out all three enzymes o Concluded that DNA coding sequences for all three enzymes are linked as a unit on the bacterial DNA and are controlled by a common mechanism Every enzyme-coding sequence is a structural gene o Operons are gene complexes consisting of a group of structural genes with related functions plus the closely linked DNA sequences responsible for controlling them Lac Operon o RNA polymerase binds to a single promoter region upstream from the coding sequences o Proceeds to transcribe the DNA, forming a single mRNA molecule that contains the coding information for all three enzymes o Synthesis is coordinated by turning a single molecular “switch” on or off
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Switch that controls mRNA synthesis is the operator Sequence of bases upstream from the first structural gene in the operon In the absence of lactose, a repressor protein (lactose repressor) binds tightly to the operator RNA polymerase is blocked from transcribing the protein- coding genes of the operon Encoded by a repressor gene which is adjacent structural gene located upstream from the promoter site Lactose “turns on” the transcription of the operon because the lactose repressor protein contains a second functional region separate from its DNA bonding site Binds to allolactose Alters the shape of the protein so that its DNA-binding site no longer recognizes the operator Jacob and Monod isolated genetic mutants to study the lac operon Divided their mutant strains into two groups based on whether a particular mutation affected only one enzyme or all three o One group, only one enzyme of the three was affected o
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